What sort of mods do you mean? Code mods?
There's a Mac Mod SDK here (
http://lucasfiles.com/?s=&action=file&id=615) (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 (
http://blender3d.org).
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)