How can I re-register a previously deregistered recipient?

Under Subscribers > Opt-outs, you will find the tab Unsubscriptions of All Lists.

Enter part or the entire email address of the unsubscribed subscriber into the search field and click the magnifying glass to start the search. In the results, click "Delete" in the Actions column to remove the subscriber's record from the unsubscriptions. The subscriber's data can then be re-imported or manually added.