If you can't see an obvious lever of some kind, try lifting the arm all the way out (away from the windshield). That should unlock the arm, and you should be able to lift it off. It could be on there pretty tight so try prying with something soft, like the rubber handle of some pliers.

'Course, I'm just guessing. Don't pry too hard! <img src="/forums/images/graemlins/cyclops.gif" alt="" />