In order to use the HP Doc, you must first be enchanted to cross the barrier. Walk East into the deadend to get to the Pillar Room. Run around the room by standing between the pillars in a clockwise fashion. South - West - North - East then step into the middle of the pillars to arrive outside the building. Continue on to the Doctor.

Climb the stairs to see the HP Doc.

HP Doc will ask for a certain crit to be killed or a rare item to bring back to receive your HP.

Some HP Doc wanted Items:
Tourmaline.
Fang.
Frog Eggs.
Levi Egg.
Bone Ring.
Ruby with small dragon inside.
Brass Key.
Lori Statue.
Helm.
Gold Bars.
Fungus Robe.