If you download them with Safari they might get renamed to map.zip. Worse still, if you just rename them the usual way they'll turn into map.pk3.zip and hide the extension .zip. Click on them, use command-I to get info, then edit the name there. Or else, just use terminal: mv map.zip ~/Library/Application\ Support/Tremulous/base/map.pk3
By the way, you should move the default maps there too, so they are accessible wherever tremulous is.