The first time you launch BuddyPop, you see something like this :

As you can see, configuration is very simple, just choose your shortcut modifiers, and type a key to associate it with, save your preference and you are done.
You can also adjust width or height if you have contacts with long email addresses or a lot of information.
When configured, you should see something like this when typing shortcut (Apple-Alt-Space by default):

The left button is used to configure BuddyPop. The right button opens the current contact, if applicable, in Address Book.

While you type, you see the number of matches in your address book. Type Return to show the full contact card.

If you get more than one match, use left arrow and right arrow to browse between them.