How to use Figma plugin with Setapp subscription?

Sign into your Mockuuups account from the Setapp version of the desktop app. It needs to be the one you download through Setapp store that says "Setapp Member" in the top right of the screen.

Then you can use the same Mockuuups account in the Figma plugin and it makes the connection that will allow you to extend your Setapp license into the Figma plugin.