If you linked your ESP account, and then added a new list to that account later on, the newly added list will appear within the "Import Lists" section and you will be able to manually import it for validation. Once imported, the validation process automatically begins on the list and you are notified once the list is done in order to view the Free Report.
After viewing the Free Report, you can add the list to your cart, checkout, and purchase the validation results for the mentioned list. Once purchased, the results will automatically update back to your list in MailChimp and you will be able to see the A+, A, B and D grades next to every email address in the list. The F’s are unsubscribed and you can find them in the Unsubscribed section of the list (should you further need them).