Try this:
If you click on the New message and To: a dialog box appears which should contain the contacts. However, what it will actually display is the Outlook Address Book, which will not have any contacts in it.
To solve this temporarily, simply choose "Contacts" from the "Show Names From the" drop down list. You may actually have more than one "Contacts" listed. Choose the last one in the list.
To change the default list, choose Tools|Email Accounts (works in Outlook 2003 not sure about others), then View or change existing directories or Address Books from the Wizard. Press Next. The next screen should list something like "Outlook Address Book". Select it and press the Remove button. Restart Outlook.
Then select Tools|Email Accounts again and this time select Add a New Directory or Address Book. Press Next. Choose "Additional Address Books" from the list given and press Next. Choose Outlook Address book from the list and press Next.
From the Folder List view, right clich the Contacts folder and choose Properties from the menu displayed. Choose the Outlook Address Book tab. Check "Show this folder as an Outlook Address Book" and press OK.
The Outlook contacts should now appear by default in the To: Address List.