I tried in vain to get GTKRadiant to work on my Mac, but X11 didn't like it- maybe because it's an Intel Mac (?). I gave up, and just boot up Windows to run it.
And the Blender .map exporter, what a horrible thing, can anybody find a good use for it?
Since it just divides up a mesh into a plethora of brushes, it's really not useful for anything intricate. I can see a use (maybe) if one wanted to make some odd terrain, but didn't want to use a model.
I don't seem to be able to create .md3 files or import already made ones into Radiant.
I've tried numerous times with help from other mappers but they end up saying that it's a problem with Mac.
Blender's .md3 export is pretty b0rked last time I checked. How I learned to do it (get models in-game) is to export a UV mapped model from blender as an .obj, then import it into 3dsMax (not a Mac app, unfortunately), apply the texture map, and export it as an .ase.
If the file placements are correct throughout the process (models are in the data/models/modelname dir, textures are in textures/foldername/texturename, they can then be imported into Radiant via the "misc_model". They import very small though, and have to be scaled up. (thanks to tZork).
Obviously, if you don't have access to a PC with 3dsMax, this can't be done. I haven't had any luck with .md3s on a PC either, but then again, I haven't tried that hard. .ase seems to work fine.
Good luck!