What sort of mods do you mean? Code mods?
There's a Mac Mod SDK here
(but the cgame part is broken). You'll need Xcode or Projectbuilder.
If you mean skins, then you can skin normally with any graphics editing program. If you mean player models/md3s, you can't make them in-game, but you can get someone else to get them playable. - you can use pretty much any 3d software - e.g., blender3d
If you mean maps, there's a forum (and installer) for the mac version of gtkradiant - http://redsaurus.net/gtkradiant
You can edit text files ok with TextEdit (preferably with plain text set in preferences)