What happened to Exposé?
In OS X 10.7, Exposé has been outmoded by Mission Control. It essentially combines Exposé and Spaces into one set of gestures. How you access it will depend on your computer and its configuration.

MacBooks with glass, buttonless trackpads and desktops with the Magic Trackpad can access it by swiping four fingers up, I believe. Older MacBooks that run Lion can access it with the Exposé button on the keyboard. And, of course, all Macs can be configured to launch Mission Control by moving the cursor to a corner. It can all be accessed via the Mission Control pane in the System Preferences.