This happens when you have installed TRPG incorrectly. Check your Tribes folder. If you don't have an RPG subfolder there with lots and lots of files in it, there's your problem.
Alternatively, you just didn't launch Tribes with the RPG mod. To do this, make a shortcut to Tribes.exe somewhere. Then, edit the shortcut to add -mod RPG to the end of the target.
For example (just don't add anything starting with where it says +connect):