mechbox.com has a video.
You pretty much just unscrew and pop out the rear plate, then whack the stock a couple of times against something and eventually the mechbox pops out. I read somewhere (I think on ASC) that you can put some para cord or something through the hole at the back of the mechbox to help you pull it out more easily.
here's arnies'
http://arniesairsoft.co.uk/?filnavn=...aug/index.html
page 3 has the mechbox removal step, but it's not entirely obvious
here's mechbox.com's
http://www.mechbox.com/site/aug-upgr...bly-video.html