A little confusing to do, but not too hard. Go to "your home folder/Library/Application Support/Quake 3/baseq3/"
In there is a file called "q3config.cfg." Open this with a text editor such as BBEdit or SimpleText. Then Change the line called "r_customheight" to 1024 and "r_customwidth" to 1600. Then change "r_mode" to "-1". If some of these lines do not exist, then just make them at the bottom of the text file. Launch Quake back up and you should have widescreen. The only problem is you can't see the id logo at the opening of quake, so don't think you messed up.