So here is how to configure GtkRadiant:
download GtkRadiant 1.4 from http://www.qeradiant.com/ and install it where you like to have Radiant installed. 4 folders have been created:
3 for different games and gtkradiant's one e.g.
C:\Program Files\GtkRadiant-1.4
C:\Program Files\Quake III Arena
...
We will have to modify the Quake III Arena configuration to use it for Nexuiz mapping instead:
Download the attached file ( I got it from http://trthome.dyndns.org:8000/Reaper/Nexuiz_ents.def ) and place it in
...\Quake III Arena\baseq3\scripts\
This way Nexuiz entities will be loaded in GtkRadiant.
One single step left:
unpack all the contents of ...\data\data20050531.pk3 in your game installation into ...\Quake III Arena\baseq3\
the pk3 file is as in all q3-based games a zip file.
Now you should be ready to roll.
Stolen From Nexuiz forums, thanks to no_dammagE