MacRadiant is a port of the GtkRadiant map editor for OSX, it exists thanks to Eridan. The only pre-requisit is that you have Apple's X11 installed.
You can get the most up to date version of MacRadiant 1.4 here
You'll also need to get the game packs. GtkRadiant supports Quake 3, Star Trek: Elite Force, SOF2, Jedi Knight II: Jedi Outcast, Jedi Knight: Jedi Academy, Wolfenstein: ET.
The original GtkRadiant is in a bundle, containing GtkRadiant 1.4 and GtkRadiant 1.5, can still be downloaded from www.redsaurus.net/macradiant/GtkRadiant_1.4_1.5_Macintosh.dmg , but it requires a PPC mac. It already contains a Packs Installer, but unless you need GtkRadiant 1.5, I recommend getting the most up to date version rather than downloading this.
Don't forget to read the documentation, and happy mapping!
MacRadiant has some issues in specific OSX versions but there are existing fixes below.
Update X11, get the latest Leopard version from here.
Navigate inside your MacRadiant 1.x bundle and locate the "script" file inside Contents/Resources. To open "Contents/Resources", just control-click or right-click on the MacRadiant 1.x icon. A menu should appear. Select "Display content" or something similar and a window should pop up, with a "Contents" folder in it.
Open the "script" file with your favorite text editor and remove the group of lines starting from the line ### Work around ...etc... down to (and including) the line export DISPLAY= ...etc... Then, you should have, at the end of the file: ulimit -n 512 exec "$CWD/sw/ .... etc ....
Open the terminal and type the the following three lines (in the following, YOURPATH should be replaced by the path to your MacRadiant application bundle. The commands are separed with semicolon (;), that means that it is a different line: cd "YOURPATH/MacRadiant 1.4.app/Contents/Resources/sw/lib"; mv libiconv.2.dylib libiconv.2.dylib.orig; ln -s /usr/lib/libiconv.dylib libiconv.2.dylib
Make all the changes above, skip the X11 updating, it isn't needed.
Open the terminal and go to the lib folder inside the MacRadiant bundle by entering the following line: cd "/Applications/MacRadiant 1.4.app/Contents/Resources/sw/lib". If MacRadiant is not located in the Applications folder, change the beginning of the preceding line accordingly.
Type the following lines and press the return key (the commands are separed with semicolon (;), that means that it is a different line): rm libxml2.2.dylib; ln -s /usr/lib/libxml2.2.dylib libxml2.2.dylib