As soon as you run the command Flx or FlexibleDuct, the program prompts you to specify the Diameter & the Number of fins. After that, you should select Lines, Arcs, Splines or LWpolylines in order to convert them to Flexible Duct.

The installer that ran when you downloaded this app/plug-in from Autodesk App Store will start installing the app/plug-in. OR, simply double-click the downloaded installer to install the app/plugin. You may need to restart the Autodesk product to activate the app/plug-in. To uninstall this plug-in, exit the Autodesk product if you are currently running it, simply rerun the installer, and select the "Uninstall" button. OR, click Control Panel > Programs > Programs and Features (Windows 7/8.1/10/11) and uninstall as you would any other application from your system.