Ok Here let me get you a link to set up Radiant to use with Open Arena. Then you will have the textures load up.
http://furver.no-ip.org/rainbow/index.php?name=Forums&file=viewtopic&t=595&highlight=radiantJust scroll down and start reading where it starts out
"You will notice that the program is working as it should (if everything went well during installation) but you only have the option to choose 'Quake 3 Arena / Team Arena' and this is not what we want right now."
P.S. If you are using vista like I am you will need to turn off user account control to be able to save the files after you edit them.