If you're using 4.0...you're out of luck. If you're using 4.5, you have to download mentalray 1.5.1 from alias' site (not sure if they still offer it). If you're using 5.0, the integrated version of mentalray comes with it and is on the install CD. Just install it and make sure all related plugins are enabled in the plugin manager.
"Terminat Bora Diem, Terminal Auctor opus."