I don't know exactly what causes this - Microsoft have a knowledgebase
article that suggests that you need to bring the "MDAC" data access
components on your system up-to-date (the MDAC is downloadable from the
microsoft web site). However, I would also check the following:
a. Are your addresses actually in Outlook (e.g. not Outlook Express)? Do
you have multiple Outlook profiles on your system? Or have you been using
any other e-mail software?
b. is the Contacts folder you want to use marked as "Show this folder as an
e-mail Address Book?" And does it have the name you expect (select the
folder in Outlook, right-click and choose the Outlook Address Book tab)?

Which version of Windows are you using?
--
Peter Jamieson
MS Word MVP