Which retroarch core to use?

Hi guys, I'm hoping someone here can help me because I'm really struggling to get Naomi/atomiswave games working.

The problem I have is that dig offers me two emulator options 1. Retroarch (reicast) and 2. Retroarch (MAME).

When I select Reicast it asks me to download the Reicast core but I can't see the core in the list to download it. I think it may have been replaced by the flycast core which dig doesn't seem to support yet.

If I select MAME, it looks like it is loading the rom but then goes to a black screen and then back to dig. I suspect either the ROMs I have aren't the correct ones. (I know where to find them, just no idea how to tell which ones contain the correct files, if they need to be zipped or not etc) or I haven't put the appropriate BIOS files in the appropriate directory. I have both atomiswave and Dreamcast bioses in the retroarch system folder but again don't know if this is the right place for them. Any help would be much appreciated. Thank you.