If you'd like to connect directly to Particle's server, you can download his Client Updater program which, after patching your game automatically (it's pretty sweet), will automatically connect you to the server. Else you can just put this in the target area of your Tribes RPG shortcut...
"..\Dynamix\TRIBES\Tribes.exe" -mod RPG +connect tribes.pcrpg.org:28001
Server numbers vary on a day to day basis. Sometimes there are a dozen at least, other times, zero.