acc_homer is a Defrag map. It can (and should) be downloaded from the Worldspawn Archives.
It's not necessary to have the Quake 3 resources to play Defrag with Open Arena. Download and install OA, download and install DF in the same place, download a custom Q3 engine (such as dfengine, or iodfengine), and create a batch file with this inside it:
It should launch DF without any problems. If it does, then you can watch the demo from there. If it doesn't, you would need to post a console log, since I know for a fact this works.
iodfengine.x86.exe -nosplash +set fs_game "defrag"