you will need to find a driver for mslug 5 and dl mame source. then add the driver to the source and compiile it. if you look on the mame homepage you will find the tools that you need. also if you do a search on google you will find a driver ,search for mame metal slug 5 driver. and go to emuchrist for a nice tutorial on how to compile.you must make sure your rom files match the drivers.
you can also add other games this way and you can enable the disabled games as well.
the reason for my previous post was because mame doesnt want people to add games. they have a 3 or 4 year rule. so if a mod sees this and it is against the rules please delete it. thanks.