I assume if you just moved to mac, you now run with Lion.
So, about the primary folder you downloaded ( where you find the app launcher ), just put that folder in the applications folder.
Your config file ( the " real " baseoa ) is located in your applications supports, to find it, you got 2 ways ( as it's not directly shown on Lion whereas it was before ):
1) open finder
2) Select " Go " in the taskbar
3) then you got 2 possiblities, press alt and it'll show up the library, or click on " Go to folder... " and then write Library in it.
4) Once you're in the library, just get in " Applications supports "
5) There find the Open Arena file, and you'll find the baseoa folder in there with all the maps/configs/mods and such
About the video, don't use your old windows settings, just get in the vids settings and put everything to the highest, personally i don't have the 27" imac, i got both an old 24" and the newest 21,5", and on both i got complete full screen with the highest settings
If you got any questions for mac, you can ask me if you want