Michele Marcelli at the ScriptSpot has a free Quick Ambient Occlusion (Mental Ray) 1.5 script. He says, “This simple and fun script creates a fast Ambient Occlusion. It should be used with Mental Ray. The operation is non-destructive, so you do not loose the original settings. I hope it can help you! 🙂 Enjoy!” It does require 3ds Max 2010 and up.
cgi tuts+ has a tutorial on using the script in part 3 of their article 3 Ways to Create Ambient Occlusion in 3ds Max.