To save a 3D model to 3MF file:

1) Open a 3D model.

2) Click on the 3MF File Writer button under the AddIn's menu

3) Specify an output path in save dialog.

4) Click save.

The file can be viewed in 3D builder.

Install:

The installer that ran when you downloaded this app/plug-in from Autodesk App Store has already installed the app/plug-in.

OR, simply double-click the MSI to install the app.

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™.

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, you can uninstall it from 'Control Panel\Programs\Programs and Features' (Windows 7/8/8.1/10), just as you would uninstall any other application from your system.