The MAME importer asks you for your rom path, but doesnt actually use it as such. It tells MAME what file name to load, and MAME knows where that file is, as long as you specified it to MAME. Retroarch would be a little different in that it would need the actual file passed to it in order to be loaded. It is a MAME importer for a reason, while you can of course use retroarch ( i wouldnt personally as there is no benefit at all to doing so over the standalone version) im not sure how exactly retroarch expects the files, or if it needs a rompath like standalone does.