Once the add-in is running its button can be found under the CREATE menu. 

Description of inputs 

Gear Standards:

The true involute pitch and involute geometry of a helical gear is in the plane of rotation (Radial System). However, because of the nature of tooth generation with a rack-type hob, a single tool can generate helical gears at all helix angles as well as standard spur gears. However, this means the normal pitch is the common denominator and usually is taken as a standard value (e.g. 14.5 deg or most commonly 20 deg). In other words, if you plan to have your gear manufactured with a standard hob you will likely want to use the "Normal System" and a pressure Angle of 20 degrees.

Normal System: Pressure angle and module are defined relative to the normal of the tooth (i.e. defined as if the tooth was rotated by the helix angle). When defining a gear in the normal system changes to the Helix Angle will cause the diameter of the gear to change as well as the working thickness (and therefore the strength) of the tooth.

Radial System: Pressure angle and module are defined relative to the plane of rotation. When defining a gear in the radial system changes to the Helix Angle does NOT affect the gear diameter but it does change the "normal pressure angle" which may require custom tooling to have the gear manufactured (obviously this is not an issue if 3D printing the part).

Sunderland: The Sunderland machine is commonly used to make a double helical gear, or herringbone, gear. The radial pressure angle and helix angle are fixed at 20° and 22.5°, respectively.  The tooth profile of Sunderland gears is also slightly shorter (and therefore stronger) than equivalent gears defined in the radial system.

Handedness: Direction the tooth appears to lean when placed flat on a table.

Helical gears of the opposite hand operate on parallel shafts.

Helical gears of the same hand operate at right angles.

Helix Angle: Angle of tooth twist 0 degrees produces a standard spur gear. The higher the helix angle the more twist the gear has.

Pressure Angle: The pressure angle defines the angle of the line of action which is a common tangent between the two base circles of a pair of gears. The short of it is this: leave this value at 20 degrees until you have reason to do otherwise - but know that any pair of gears MUST have the same pressure angle.

Module: The module is the length of pitch diameter per tooth. Therefore, m = d / z; where m is a module, d is the pitch diameter of the gear, and z is the number of teeth.

Teeth: Number of teeth the gear has. The higher the helix angle, and to some extent pressure angle, are the fewer teeth the gear needs to have to avoid undercutting. It is possible to create a Helical gear with a single tooth given a high enough Helix Angle. CAUTION: due to performance reasons, do not make gears with several hundred teeth.

Backlash: [experimental] a positive value here causes each tooth to be slightly narrower than the ideal tooth. In the real world having a perfect tooth is not often desired, it is better to build in a little backlash to reduce friction, allowing room for lubricant between teeth, and to prevent jamming. Backlash is allowed to also be negative which has no real world application I'm aware of but may be useful for compensating for undersized teeth which were 3D printed, etc.

Gear Thickness: How thick you want the gear to be. CAUTION: making a gear really thick can cause some serious performance issues. If you wish to make a gear where the teeth wrap around multiple times I recommend seeing the "length per revolution" field in the "Gear Parameters" readout and using that value for your gear thickness then copy/rectangular pattern the gear body to reach your desired length. This is something that may be addressed in a future release.

Preview (checkbox): When checked the full gear will be generated and updated as you change parameters. For performance reasons, you may wish to leave this unchecked - especially for gears with over 100 teeth.

Gear Parameters: Presents a readout of all significant gear parameters calculated for your specified gear. It is advised that you check this section for a warning of UNDERCUT REQUIRED. This tool does not perform tooth undercutting at this time! If you encounter UNDERCUT REQUIRED you can do one or more of: increase the number of teeth; increase the Helix Angle; increase the pressure Angle. 

To create gears that mesh at nonparallel and nonintersecting axes the sum of their helix angles must equal the shaft angle. You can treat right-hand gears as having a positive angle and left-hand gears as having a negative angle. There are many solutions for any given shaft angle E.g. 22.5(R) + 22.5(R) = 45; and 65(R) + -20(L) = 45

Install:
Double-click the downloaded installer from Autodesk App Store to install the app/plug-in.

Uninstall:
You can unload the add-in without uninstalling it by clicking the "Stop" button when the add-in is selected in the list box on the "Add-Ins" tab of the "Scripts and Add-Ins" dialog.
Unchecking "Run on Startup" causes the add-in not to be loaded in future sessions of Autodesk® Fusion 360™.
- Navigate to the ./ApplicationsPlugins folder and select the app bundle you want to uninstall
- Either drag the app bundle to the Trash or right-click and select “Move to Trash”
- Right-click on the Trash-Can and select “Empty Trash”
- If you prefer keystrokes, you can also just select the app bundle and then hit Command+Delete to move the app to Trash, then empty the Trash and the app will be removed.

As an excellent resource for information about all types of gears I highly recommend this Tech document by qtcgears.com. This document was used as the primary authority for the math and theory behind this add-in.

Performance becomes an issue when teeth wrap around the gear multiple times - don't make long screws with this add-in and you should be fine. It can also be helpful to uncheck the preview checkbox when setting up complex gear. 

Performance is also an issue when creating gears with more than a couple hundred teeth - save your drawings before attempting! 

Be sure to expand the Gear Parameters section and check for UNDERCUT REQUIRED warnings. This tool does not perform tooth undercutting at this time! If you encounter UNDERCUT REQUIRED you can do one or more of: increase the number of teeth; increase the Helix Angle; increase the pressure Angle.