If you're having trouble getting Lion to let you enable Find My Mac, you're not the only one. It seems at least a few people are running into a problem where they see the above message in the iCloud preference pane, but clicking the update button (or running System Update manually) yields no updates to install. If this describes your situation, the good news is that there is a solution -- the bad news is that it might require a little bit of patience.
Before getting too worked up over the problem, though, it's a good idea to try the simplest methods first. Go to System Preferences, go to the Security and Privacy preference pane, and click on the Privacy tab. Check to make sure that Location Services are enabled:
