make sure you put the space between tribes and -mod rpg
also there is a space between mod and rpg.
If you really wanna solve the problem I would advise making a backup of your config and then re-installing tribes. Follow the rpg installation instructions to a T. It may be that you didn't name a folder "rpg" or that your directories may be f@#cked up. Make sure your rpg folder is in the same directory as your config.....meaning, when you go into "my computer" then your destination drive...C or D or whatever, then Dynamix, then Tribes, your rpg folder should be sitting in that directory with config and all that other stuff. If its not in that directory, then put it there and try again.
Also, try making a new shortcut because if you're not using a shortcut, then it won't work. Just make a new one and add that -mod rpg and see what happens. It doesn't hurt to try. Keep me posted on this.