This has been asked before but we were told there's no connection between Sales Reps who sign up a merchant and the forum.
Why not add a statement to the new merchant's Welcome Email that would advise them of this. Something as simple as:
"Now that you have an eccommerce site with MonsterCommerce, we want you to have the best experience possible. Please contact Tech Support by phone, xxx-xxx-xxxx, or by submitting a ticket to have your forums permissions changed to include the Merchant Only forums, where you will find much helpful information about designing and setting up your new store."
That wouldn't catch everyone, but possibly most new store owners.