wow this has been covered extensively here..but I know it is always a pain in the tail...
Using the genesis gx plus core..(the pico core WILL NOT work) make sure all your controllers are plugged in and are configured first!
Start the game... once game is loaded go the the retroarch menu by pressing select+X). in the QUICK MENU go to the controls menu. for player 2 (Port 2) set it to MD 6 button + 4-Way play (Teamplayer will work here oddly enough) and then back out. on the controller main menu select "Save game remap file". quit retroarch. reload the game..then test it (with friends of course!)Problem is there are two type of multi-taps for the Mega Drive/Genesis. One for sports games made by EA sports (called Teamplayer)and another made for "general four player (non sports) games". Normally that means the 4-way play. It is hit and miss on the game..some wanted it in player two..some wanted it in player one. But it works for this game at least just like I typed above.
Happy gaming! :D