How do you pry apart pieces stuck together by cam screws?
There is a clearance hole for the screw portion.  The pieces slide together and it is the cam that locks it.  There may also be some dowels or metal pins for additional strength.  If it won't come apart, something is binding (unless the pieces or dowels were glued).  Try this:


Stick something into the bottom of the cam hole to protect the MDF, then use a large screw driver between that and the screw head for leverage to push it up.  

There are likely several cam locks, plus potentially some dowels; the pieces will need to remain square with each other to separate them.  You won't be able to separate them by pushing on just one screw.  Put pressure on one screw to try to break apart what's binding them.  Then do the same on the other side.  The idea is to separate the pieces as evenly as possible.  If you jack up just one side, everything will bind.  Once you get it started, it will become easier to pull them apart.
If you can create even a small gap in one spot, push a piece of sheet metal into the gap (if you don't have any laying around, cut it out of a soda can).  Any place you can create a gap, add a piece of sheet metal there.  That will maintain some tension while you work on another spot.  But don't stick a bunch of shims in just one spot; keep it even across the joint.  

If you have wood or plastic shims, use those instead of the sheet metal.  Separate the pieces enough to get the tip of a shim between them.  Try to get a shim in the same location at both inside and outside the joint so you don't push the pieces out of perpendicular.  Once you get shims started all along the joint, gradually and evenly tap them into the joint to push the pieces apart.
Once you get a little gap between the pieces, if they are still hard to pull apart, use a pry bar between the pieces for extra leverage, working it in different spots across the joint, a little at a time.  Don't use so much pressure that you damage the pieces.  
It may also help to whack the underside of the top with a rubber mallet along the joint (more useful if you can lay the unit on its back so gravity isn't working against you).  In fact, you might want to start with this as a first step.  If you don't have a soft mallet, protect the underside of the top with a board and whack the board with a hammer.  Don't use excessive force.  If it's glued, a big enough hammer and enough force will probably separate the pieces, but the result won't be pretty.


If none of the above budges the pieces, it probably means that glue was used.  In that case, you aren't likely to separate the pieces without hunks of MDF breaking off.