Protheus:
The mental ray 1.5 is for Maya 4.5 only, as I recall.

taruqrf:
I think you want to get to mental rays final gather and global illumination menus.

These menus does not appear until you change one of the first options in the mentalray render globals (I assume you have found this menu by now).

One of the settings says "derive from maya", this you must change to either "production", which gives you new menus with some defaults that gives some "better" rendering "quality".

You would want to change it to say either "preview final gather" or "preview global illumination" or "preview caustics" (never used this last menu's defaults).

um, so there are the menus, ask away if you have some specific questions.