When you imported the contacts, did it create a new contacts folder? E.g. if you go into People and look in the left hand navigation bar, it will say "My Contacts" and underneath will list any contacts folders you have within your profile. Default folder is called Contacts. If you have more than one then you often have to manually select that you want the additional contact folders to be used for email.

Select the one which contains the new contacts you have imported, right mouse click on the folder and choose Properties. Select the Outlook Address Book tab and then tick the box for Show this folder as an e-mail address book. Click OK. Go back into a new email message window and click To, the new contacts folder should now be displayed in the drop down list allowing you to choose a contact from it.

Let me know if you still have problems.