First you need to enter the masterlist in your account.
You will find the masterlist at the top of the Recipient section, it's called @All Addresses:
Once you enter the masterlist it will show by default all of your subscribers on the account.
Unsubscribed list (opt-out all)
Change the selection to "Unsubscribed (Opt-out all) and click search:
You can also type in a search parameter in the search field to help yourself narrow down the search of the subscriber. For example, choose Name/Email to search for a specific email, or even all emails containing certain elements, like @gmail.com.
Resubscribe and unsubscribed profile
After you have found the profile, click on the red cross and choose 'Delete' to remove them from the account. Once they have been removed from the account, then they can be added again as a subscriber.
NOTE:
Due to the importance of the list there is a limitation that is preventing the user to delete many ex-subscribers at once, it has to be done one-by-one.