When you buy Mainstage 2 from the App Store the loops and instruments will be installed in the system library
/Library/Application Support/GarageBand/Instrument Library/
That are the same places that are used in GarageBand. So no extra steps should be necessary to use them in GarageBand. Maybe you will need to rebuild the Loop Index - that is done by dragging the added loops onto the loop browser in GarageBand.
The installation will proceed in two stages: First the Mainstage application will be downloaded, and then you will be prompted to download additional contents - more loops and instruments.