Wee, Fun Stuff! This Is Gonna Be Long
Sleepwalker: the short answer:
http://redsaurus.net/gtkradiant/ <-forum
http://www.redsaurus.net/macradiant/GtkRadiant_1.4_1.5_Macintosh.dmg <- should be universal binary installer
The long answer is, the short answer probably doesnt't help much at all. Half that forum is outdated information, the other half is info about useless games like Jedi Academy. The installer and some useful stuff is on there somewhere, though. I'll start from the very beginning because I don't know how far you got.
I am on PowerPC/Tiger and I suspect you are on Intel/Leopard, which is a minor hurdle but not insurmountable. I did have it installed on 10.5 for a while, but this old laptop just didn't get along with Leopard too well.
Install X11 first, nothing will run otherwise. In Tiger this came on the install discs, on Leopard get the latest from
http://xquartz.macosforge.org/
OK, inside the DMG I linked to, you find GTKRadiants 1.4 and 1.5, and a Packs installer. There is some documentation in there, but it's not incredibly helpful. Run the Packs Installer; there is no Q1 pack, but it needs to find at least one game to install some files that Radiant needs, so point it at a Q3 installation. Put the Radiants whereever you want on your HD, it will just change the file paths we'll be editing later. I personally hate gtkr 1.5 with a passion, so everything that follows will be for 1.4.
Load that bad boy up and see if it runs. Oops, it almost certainly doesn't. You'll need this:
http://www.redsaurus.net/macradiant/MacRadiant14intel.dmg
For more info on getting that to run, I'll refer you to:
http://tremmapping.pbworks.com/GtkRadiant1_4-Intel_MAC
http://openarena.wikia.com/wiki/Mapping_With_MacRadiant
http://alientrap.org/forum/viewtopic.php?p=37611
I don't have an Intel Mac (yet), so I hope that helps enough to at least get the thing to open. The NetRadiant fork may be easier to get up and running, but it's 1.5 based, so bleh.
Quake 1 support section! Once it at least runs, close it again, and do View Package Contents -> Resources -> sw -> radiant14. Hey look, it's the actual Radiant folder, buried in the app package. From here on, the process parallels what Tigger-On describes here, configuring Radiant for Quake:
http://industri.sourceforge.net/howto.php
find the games folder, copy q3.game and name it q1.game, change some stuff. Mine looks like:
<?xml version="1.0" encoding="iso-8859-1" standalone="yes"?>
<game
name="Quake 1 / Nehahra"
gametools= "/Applications / Games / Quake1 / pathofthesoul / MacRadiant14.app / Contents / Resources / sw / radiant14 / q1"
basegame="id1"
enginepath= "/Applications / Games / Quake1"
engine="Fitzquake.app"
prefix=".q1"
default_scale="1"
/>
Adjust accordingly. I added those weird spaces just now so the forum doesnt freak out, but you get the idea. Notice I have "Quake1" not "Quake 1" though, I might have done this because because Radiant didn't like seeing any spaces in its file path.
Now you can put stuff into id1/textures, id1/scripts, id1/maps, etc., just like on PC, yay. I later tried to get clever with project files and point it to a mod directory parallel to id1, which kinda works.
Ah, one last thing. every time you try to open or save a map, the file dialog probably isn't pointing to where you want it, and the GTK file dialog box is pretty shit. In your home folder, there is a hidden folder with the name given in the "prefix" line of the
.game file, usually this is ".q3a" but I changed it to ".q1". Inside should be a folder with the name of the basegame directory, which probably will be "id1." Inside that folder, make unix symlinks to your actual working maps and scripts folders. The documentation file in the .dmg we downloaded has a simple method of finding the path, and a contextual menu plugin to make the symlinking easier.
It's worth noting, Sleepwalkr, that I'd have given up at this years ago if it weren't for your java mapconverter - works like a charm in OSX! So thanks for that.