Deposits and waitlists are not available for use with conferences because of the complex structure of this module.
If you want to have something that mimics a conference waitlist, you can create separate courses and classes for each conference session. You might use a course group called Camp Waitlist or similar. These classes would be used with capacity at zero and are would simply be used to build waitlists for those individually popular sessions. For each conference, you could add a message and link to these class(es) that lets students pick something in the time slot to ensure they have something but then also get on a wait list for the specific session that is full that they really want in that particular time slot.