Well, it's been a long time, but artwork has changed formats since about the 0.107 era of MAME (last MacMAME was 0.103u2) - so, if you have artwork for MacMAME it will not be detected or work in current MAME.
Good news, though, there is a plethora of artwork available for current builds at a quantity and quality much better then every before.
http://mrdo.mameworld.info/mame_artwork.html