(EDIT - Info on how I put aerobars on my Mountain Bike below the photos)
This post has been really popular for people looking for information about installing Aerobars on a mountain bike. What I did is rather simple. First, I traded in my riser bar for a old, cheap, aluminum flat bar. Typically, carbon mountain bike bars cannot handle bar ends, so I highly doubt they will be able to take aerobars.
I decided to use the Profile Designs t2+ Cobra because they were both light and compact. I didn't have to shim them at all, however using some old tubes probably couldn't hurt.
