To open the add-in window, choose Level Surface Mesh, which can be found under 'Create' menu in the Model workspace.

When the add-in window opens, it will display a live preview of the mesh based on the default settings.

As you change these settings, the preview will be updated to reflect the mesh that will be created in the current document when you execute the add-in.

Install:
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.

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 and Features' (Windows 7/8.1/10), just as you would uninstall any other application from your system.