First you'll need to create a Custom Island (CP -> Portal -> Custom Islands), then you'll edit the forum details (CP -> Content -> Forums -> Details) and select the Custom Island as an Ad Island under "Custom Island Insert" in the "Custom Inserts" tab.

Now, you'll need to set the permissions for "Must view Ad Island" from the Forum Permissions (CP -> Content -> Forums -> Permissions or CP -> Permissions -> Forum).

I am a Web Development Contractor, I do not work for UBBCentral. I have provided free User to User Support since the beginning of these support forums.
Do you need Forum Install or Upgrade Services?
Forums: A Gardeners Forum, Scouters World
UBB.threads: UBBWiki, UBB Styles, UBB.Sitemaps
Longtime Supporter & Resident Post-A-Holic
VNC Web Services: Code Modifications, Upgrades, Styling, Coding Services, Disaster Recovery, and more!