Sys_Error: Couldn't load default.cfg / Sys_Error: FS_FreeFile( NULL )

If you get one of these errors after installing Quake III Arena or Return To Castle Wolfenstein, it means that the engine didn't find all the required .pk3 files. Either you didn't copy them, or you copied them to the wrong place. Check the INSTALL instructions for the game for more details, make sure they are in baseq3/ for quake3 (missionpack/ for TA files) and main/ for Return To Castle Wolfenstein

http://zerowing.idsoftware.com/linux/q3a/