Sorry about the lack of documentation. I’ll take care of that this week.
First, enable Extended Profile Fields support:
Admin > Settings > bbPNNS/BuddyPress Bridge > Use BuddyPress’ Extended Profiles to control Forum Digests settings in the front-end. > Save Changes.
Then add the profile field via BuddyPress:
Admin > Users > Profile Fields > Add New Field > Type > Special Fields / Forum Digest Options. Fill in the name and description and whatever other options you’d like, and click Save.