Be friendly with other interviewees and the d3 and d4's that are giving your tour. Be talkative (intelligently, not annoyingly). And show your intelligence and your people skills.
Know a little about the school. Have questions to ask your interviewers. Know your stance on dentistry, ethics, studying/school, etc. Know why you want to be a dentist and why you think you are prepared to become accepted into dschool.
It's pretty laid back, no big deal; don't be too nervous.