You won't get the full multi-touch features, but it will work as a basic trackpad-mouse.
Bluetooth icon (upper right of screen)->Set up Bluetooth Device... (or run "Bluetooth Setup Assistant")
Continue... Select Device Type: Any device
IMPORTANT: On the Searching for your device screen, you must click Passkey Options... and select Use an "empty" passkey.
The Magic Trackpad will show up as Apple Wireless Trackpad, you just select it and it will pair and that's it.
(It will ask you if you want to use it as an input device, just say yes.)