Is slop in the headtube normal?
That's not normal - any knock is also going to accelerate wear so you must fix it soon.

Did you loosen the stem bolts before tightening down the headclamp?
See [1].  The tightening order is the same for all materials, but the torques are vastly different. The torque range is usually 4-8 Nm for the head clamp for carbon forks, similar for stem clamps. You can use a special carbon grease to increase friction to that you succeed at low torques.  See [2]

Does it have a couple of years or more of wear on it?  Head clamps wear out.  Head clamps for carbon frames are remarkably skimpy things.  

[1] http://www.artscyclery.com/learningcenter/tightenyourheadset.html

[2] Attaching a stem to a carbon steerer: how much torque?