DO NO PULL OUT THE BOLT. If you do the that side of the penthouse will collapse and you could be injured.
The bolts are always under compression from the two cross members. To relieve the compression you need to raise the top with jacks and block it into position. This may not relive all of the compression but it will help.
The nut came off of mine after I first got it. With mine the bolt was still partway inside the inner tubeI took so I took a hammer and whacked the bolt back through the inner tube till the threads were visible and put a new nut on it. The nut had a nylon insert.
This video, http://vimeo.com/m/61317178
, shows how to block the penthouse up to work on the springs and bolts.