MSU-1 games are modded SNES games. AFAIK the only emulator in Recalbox that supports them is "lr-snes9x".
The short version is that you need to put the files in the "roms/snes" directory, then update your gamelist. After that, configure that specific game to use the "lr-snes9x" emulator.
There are full guides to MSU-1 on the Zelda MSU-1 site and on the Retropie site. I don't know if I'm allowed to link to them, but you should be able find them with a search engine.
Good luck. 🙂
