I am actually retiring the tool since Mister now supports MRA files that remove the need for pre-building ROMs. If you update to latest you can just place your MAME zips inside a mame folder. See this video
for more details on how it works. For some reason I can no longer update the first post to add this information :/
Regarding Galaga, it is one of the games that will give everyone a hard time. I think the creator used an old ROM version or something as its hard to get it to match up correctly. My only advice is to search for the zip name and try various ones.