Pretty simple.
You download the 1.27g update, avaliable from your favorite game file mirror (fileplanet, macgamefiles, versiontracker's link, direct from id software, whatever) and install it. Then create a folder called missionpack in your quake3 folder and copy the pk0.pk3 file from the PC Team Arena CD to the missionpack folder. I think that's it.
Rufo