Robert,
While in the Preferences dialog and the Contact Sheet section, choose the number of labels that you want and then click the "Variables..." button at the bottom of the Preferences window. A floating window will appear that lists all of the available variables. Clicking on one will provide a description at the top of the window. Double-clicking one one will paste the variable in the text field containing the cursor. (Make sure to click in one of the label fields first.)
HTH,
-Kirk