I got it to work.
Textures
I’ve unpacked the models from mine, since radiant won’t read models from the .pk3, but not textures. Sheesh…
Yes, but the question was not how to deal with models, rather how to add textures. 
We were talking about “unpacking pak0.pk3”.
Basically you just open pak0.pk3 with WinRar or PakScape and extract the model folder to your etmain folder.
Only do this when you have sufficient space on your hard disk and when you actually want to use those models. (Make sure you know how to use them first, maybe read some tutorials on using models?)
My brother doesn’t have any textures for radiant, and he has downloaded the game for it. Any suggestions?
How I was talking
unpack pk3 means -
rename pak0.pk3 on zip extract all to
Program Files/Enemy Territory/etmain
it extract all textures and all models
