Locate a compatible ROM* and put it in the /roms/ folder of the extracted client.
Current Required Compatible ROMS: Fire Red v1.0 USA
Current Optional Content Compatible ROMS: Emerald v1.0 USA
Current Optional Visuals Compatible ROMS: HeartGold, SoulSilver
* You must have the legal right to use that rom. We will not supply you with the roms, or help you find them as they are copyrighted.
STEP 5: Windows: Run PokeMMO.exe
Linux: Run PokeMMO.sh
OSX: Experimental info can be found here