When I recently replaced my image transfer kit on my office HP color laserjet 5550dn printer, I had a slight problem. The left bracket that holds the transfer kit got stuck in the upright locked position instead of being at a 45 degree angle facing out (as the instructions say it is supposed to be).
You can see this easily in figure 7 on this page:
(The above is a copy of the instructions that come with the Image Transfer Kit. Note the small circle on the left.)
I ended up calling technical support (luckily I had a service agreement, otherwise the call - productive or not - would have cost $39) and the technician there was very helpful. Here is the highly technical solution he gave me that worked.
Warning: This worked fine for me with a delicate touch and patience. I take no responsibility for the way you approach the same problem
Get a flatheaded screwdriver with a relatively long handle and not too wide a head.
Stick the head of the screwdriver in between the silver axle/bar and the front of the left bracket/cup with which you are experiencing difficulty.
Move the screwdriver (and in turn the bracket) back and forth with with enough tension to move it but not enough to break it. (this takes about 3 minutes)
At this point your locked bracket should be unlocked and at a 45 degree angle. Now you can put the new transfer kit in without difficulty.
This information should be in the instructions. The technician said that this happens so often that he had taken a picture of how to do this last week so he could send it to customers when they called. I also found several references to this problem on the web when I was looking around for a solution on how to fix this without breaking the printer.
HP tech support was very helpful in this case. But since this seems to be a rather common occurrence, I suggested there should be something on their webpage.