If you have Contacts in Microsoft Outlook's Contacts folder but they are not accessible when you click on the To button, check these settings:
Are you using the "wrong" view?
Is Name Only selected at the top of the Address book dialog? When More Columns is selected as the Search option, the address book will only display names if you search.
Is the Contact's folder enabled as an Address Book?
Make sure the Contact folder is enabled as an email address book.
To do this, right-click the Contacts folder, choose Properties then Outlook Address Book.
Is the box to Show this folder as an email address book checked?
If this is grayed out...
...and you use Outlook 2003: Go to Tools | Email Accounts, choose View or change existing directories or address book. Is the Outlook Address Book present? If it isn't listed, add it and close and restart Outlook. If it is listed, then remove it and close then restart Outlook and repeat these steps to add it.
If you are using Outlook 2007 and up and the checkbox is grayed, you'll need to remove the email account and re-add it or make a new profile as you can't remove the Outlook Address Book service in these versions.
Remove Address book (Outlook 2003 and older)
Go to Tools, Email Accounts, choose View or change existing directories or address book.
Is the Outlook Address Book listed?
If it isn't listed, click the Add button ( in second screenshot) to add it. Close and restart Outlook.
If it is listed, then remove the Address book by selecting it then pressing Remove ( in second screenshot). Close Outlook and restart. Go back to this dialog and Add the Outlook Address book service ( in screenshot).
In Outlook 2007 and up, the address book service isn't removable. You need to either remove your account and re-create it or make a new profile.
In many cases its better to make a new profile because a missing address book service means the profile is corrupt. This is especially true if you used Windows Easy Transfer to move the profile from one computer to another. Taking the time to make a new profile now may save you a lot of time in the future.
Go to Account settings, Email tab and select the account. Click Remove. Click Add and recreate the account.