Frost forming on the outside of the cryogenics chamber indicates a leak in the system and can potentially be very damaging to the system and the sample. First, turn off the compressor and slowly heat the chamber back up to room temperature. Possible causes:
Water inside the chamber due to not applying the vacuum long enough. Pressures inside the chamber should be around 10^-6 Torr or less.
Improperly closed vacuum valve: check to see if the the vacuum valve was sufficiently closed.
Insufficient seal: clean all of the seals, especially the bottom of the stainless steal casing covering the apparatus. Dust and dirt will often create difficulties in creating a vacuum seal inside the chamber. Also, clean the o-ring, being careful not to damage it upon removal (it is best not to use blunt objects or metal tools to remove the o-ring. A tiny amount of vacuum grease may then be applied to the o-ring. (The o-ring does not need more than a small pinch of grease and grease must be spread evenly on all parts of the ring).
Hydrogen leak: it is possible that a hydrogen leak has occurred (see Dr. Tate for dealing with a hydrogen leak).