Install the add-on from Host > Extensions just as you would with any other extension. Then you should be able to use either of the Neverbounce Email Verification Add-on features:

Before using any of the actions above make sure you run in the same module (eg. on Form’s init) the Setup Neverbounce Email Verification action; the action is used to set the API username and api key needed for connecting to the Neverbounce Service.