A common misconception is that BIOS files need a separate folder. They do not. Place your BIOS zip files in the exact same roms directory where you store your standard game zip files (like mslug.zip for Metal Slug ). When you launch a game, MAME will automatically scan the shared folder, find the matching BIOS zip, extract what it needs in the background, and launch the game. Step 4: Configuring Custom Paths (Optional)
Make sure the BIOS file is in the roms folder, not the main folder. Also, check that you did not accidentally unzip the file. mame bios roms 0 147