The key to Chompers Dental in the Medical Pavilion is acquired by using Telekinesis through the broken window to the left of the door. There should be a corpse hanging over the broken window. The key should be grabbed with Telekinesis because it is hanging on a wall on the other side of the window. You can get Telekinesis in an area near here. You will need to get that first, but do not follow the arrow back to the rubble that you need to get rid of until you explore Chompers Dental. There is a safe and other good items to be found inside. Note that both Telekinesis and Chompers Dental are near where you find the shotgun.