Hi,
ive got a bit of a problem, i cant remember how to turn mentalray on.
last time i had to go through a menu to enable it to be in the drop down box (containing software/hardware options), but its been so long since i done it, i cant find it again 
thanks
Well, actually you first have to set it on from the plugins menu.
Go to Window > Settings/Preferences > Plug-in Manager...(A window will come up) and you will enable a plugin called Mayatomr MR means Mental Ray. Then you will find it in the drop down window in the render globals. Good luck hope this helped.