Go to Tools | E-mail accounts, select "View or change existing directories
or address books", and click OK. If you don't see the OAB in the Directories
and Address Books list, click the Back button, then select "Add a new
directory or address book", then "Additional Address Books", and add the
OAB.  Then keep clicking Back until you get back to the first dialog box,
and go back to the Directories and Address Books list as you did earlier.
Once the OAB appears in the list, select it and then click Change to make
sure the Contacts folder(s) you want to display are listed.  If any of them
aren't listed, you'll need to enable those Contacts folders as Outlook
Address Books by right clicking the folder, selecting Properties, clicking
the Outlook Address Book tab, and checking the "Show this folder as an
E-mail address book" box.  Make sure you restart Outlook after making these
changes.