Thank you for your answer
I have made progress : I moved sg.game where the other games' packs were installed, and macradiant now shows Smokin Guns in the game list when I open it. But when I select it, it shows an error message, which says : "Synpase server initialisation failed : no such file or directory. I opened sg.game with a text editor, and tried to change the paths by looking at how it worked on Ennemy Territory : in enginepath, I put the path to the game; but I don't understand what I must direct gametools to, although I made a few trys. But for now, nothing works. Even if I'm on a mac, does anyone knows how this file should be edited (i.e what paths are supposed to lead to)? I also noticed that an Ennemy territory file contained two files named game.xlink and synapse.config : could it be related to why it is working, considerating I have no such files for SG?
It's a bit frustrating not to be able to map yet in spite of my motivation, but I'm confident I'll manage to make it work, even if it's a bit long.
When it will be the case, I'll post a quick indication on how to set macradiant, so that it will be easier for other mac players who would want to map (I just hope I'll actually make it work).