I had a teacher from the middle school run in to problems getting these really awesome science simulations from PHET Colorado to play on their MacBook Air. It's actually pretty simple to get it to work once you see the process. So here is a screencast to walk you through it.
I had a teacher ask me if there was a way to hide or remove students who are no longer in your class. The answer is maybe. It depends on if they are officially dropped from your class and appearing as red in your roster and gradebook. If not, then no, you cannot remove them until they have been unenrolled from the class. If the student is showing up as red in your gradebook though, you can hide them so you don't have to worry about them. Here is a screenshot to help you do it: