These instructions assume you’re only worried about setting up this core and have no other configurations set up.
The first thing is to get your keyboard mapped onto the retropad virtual gamepad abstraction in settings > input. So, go to ‘hotkey binds’ and make sure none of the keys you’re wanting to use have a hotkey assigned to them. If they do, press the ‘del’ key on them to unbind.
Next, go to the User 1 Input Binds menu and map your keys. Genesis Plus GX puts Genesis A on retropad Y, so you’ll go to the retropad Y entry, press accept/ok/enter on it and then hit Z on your keyboard to map it. Repeat for the rest of the keys:
Genesis B is assigned to retropad B (so map retropad B to X on your keyboard), Genesis C is assigned to retropad A (C on your keyboard), Genesis X is assigned to retropad L, Genesis Y is assigned to retropad X and Genesis Z is assigned to retropad R.
Note: this will screw up your other cores’ controls if they were already set up differently.
If you already had your retropad mapping set up on those 6 keys and just need to move the Genesis functions around on them, just go to quick menu > controls (with core+content loaded) and use the dpad to cycle through the core functions on the right until they’re on the retropad buttons you want.
Does that make sense?