Once Butter Motion is installed, it lives as a panel inside Premiere Pro. Here's how to open it.
If you signed in inside the Butter Motion app before installing, the panel opens already signed in, you'll see your credits balance instead. If you see Welcome to Butter Motion with a Sign in button instead, that just means you need to sign in again. See Sign in & connect a device.
If you don't see Butter Motion under Window → Extensions:
If it's still missing after a restart, follow Panel missing in Premiere for the full fix.