Hello. A few questions here ...

How do I go about compiling a Nexuiz build on Mac OS X?
Should I download the svn directory or use the source zips that come with the binaries?
Also to edit game variables, should I edit the Dark Places engine or the Nexuiz game data?

I'm trying to build a different, slower / smoother version of Nexuiz for my friends and I to play, so any help would be greatly appreciated.

I don't think you will need to change code for that... it probably will suffice to just adjust some variables using a cfg file. For this, go to the folder ~/.nexuiz using Finder's "go to folder" window, then go into data, and then make a new text file "autoexec.cfg". You can add settings in there, and the game will use them.

You may want to start out with

slowmo 0.7

in that file.