I would recomend 1.4 version of Gtk for ET:tcelite .map editing over the newer 1.5 version.
1.5 can handel doom3 but is still very buggy compared to 1.4
http://www.qeradiant.com/
enemy territity 1.4 binaries
http://www.qeradiant.com/index.php?data=files&files_id=76
if you dont have it already pakscap is good to have also.
http://www.fileplanet.com/dl.aspx?maldavria/pakscape/pakscape-011.zip
http://eliteforce2.filefront.com/file/PakScape_V_011;1327
you can find all the sdk matterial in the tcelite game folder
gtk will set up the folders for you but you have to install the files into the folders from tcelite .
read the mod instalition instructions that come with GTK when you instaill it.
with GTK 1.4 installed start it then hit f1 . or go to the help tab and look for gtk manual . and open it from there.