1) Do you have a skin attached to the hmp?
2) Check if the INVISIBLE (or transparent) flag is on.
3) Are you sure you havent flipped the normals (= texture side in MED!)
4) That you (the player) looks on the hmp from above in the game (since one side is transparent)!
5) Do you have the LEVEL MAP (checked) in build?
Does other things appear in the level as they should?