Not all rendering process are multi-thread. In this case, as far as i see in your screenshot, Mental Ray is in the "Translating scene" stage, which is a single thread-process (it sends the scattered items to the render engine).
Anyway, this step should be almost inmediate, even in complex scenes. For example, the final scene of the Modern Barn
tutorial doesn't take more than one second in my computer (i980x, Max 2014, FP 4.1.5).
Please check if there is some huge Forest object in your scene (you can use Forest Stats to see the estimated number of render items). What Max and Forest version do you use ?