Outside, rain pattered against the window. Inside, the virtual coin slot clinked one last time.
This usually means your ROM set is "split" or "non-merged." For the best results with OpenEmu, look for "Non-Merged" MAME ROMs openemu metal slug
Drag your mslug.zip (or mslug2.zip , etc.) into the section of OpenEmu. Outside, rain pattered against the window