Do they have an e-mail address in their contact record?  Do you see other
entries from your contacts list in that address book?

If you have Contacts in the Contacts folder but they are not accessible when
you click on the To button, check these settings:

Make sure the Contact folder is enabled as an email address book. Right
click the Contacts folder, choose Properties then Outlook Address Book. Is
the box to enable as email address book is checked?

If this is grayed out...

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.