Fusion 360 does not recognize add-in

[synthc]

I downloaded the nightly build, put it in %appdata%\Autodesk\Autodesk Fusion 360\API\AddIns and renamed it "UGS_Fusion" then restarted Fusion, but the add-in doesn't show up in the add-ins menu. I tried manually adding it (with the "+" button), but Fusion says, "A single script or add-in cannot be identified in the selected folder. Please organize your programs so each script or add-in is in its own folder."

I'm running Fusion 360 Personal 2.0.10940 on Windows 10.