How do I re activate a contact that has already been unsubscribed?

To re activate unsubscribed users simply follow these steps.

 - Click the menu item 'Contacts->Search Contacts'
 - Select the radio option 'Specific contacts from within the selected lists below' 
 - Either select '--All Lists--' to show all unsubscribed people or select the contact list that you are after for a more refined list. Click Next
 - On the filtering options page select 'Status->Unsubscribed'
 - Select Next.

That will take you to a page that will display all the unsubscribed email addresses for your contact list.

To resubscribe them simply select the 'Edit' option corresponding to their email address and change the 'Status' to 'Active'.