Press Shift+S in Fusion 360 and go to the Add-Ins tab. Then select the add-in and click the Run button. Optionally select Run on Startup.

A warning will be shown every time a new feature is created at the top-level or in the wrong component.

At that point, the following choices are given:

• Cancel (Esc): Cancel the operation
• Continue (Enter): Go through with the operation and create the feature
• Stop warning: Go through with the operation and stop warning for this document for this session.

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 7/8.1/10), just as you would uninstall any other application from your system.