You can find the company's contact information on their business profile page, and you can contact them from the ActivityHero "contact business" button. Companies should have an email and telephone number listed which you can use to directly ask about waitlists and other matters.
Typically, a camp or class will notify you by email if a spot is available. The email may be an invoice from ActivityHero inviting you to pay now in order to confirm a spot.
If you would like to contact the camp or class to ask about your status on the waitlist: