It's one of the production shaders hidden by default. You can unhide it by editing the "mentalrayCustomNodeClass.mel" found in the "mentalray\scripts" folder in the Maya install directory. Make a copy of it to your scripts folder where ever you keep your Maya user settings folder "C:\Users\Bob\Documents\maya\scripts", just to avoid messing with the actual instal folder. And edit the line the line highlighted in the image so the 0 is now a 1. Restart Maya and the production shaders are now found in the Hypershade! Mip_binaryproxy is found the geometry rollout of the Mental Ray section.

Ash did a video tut on using iton it.