The one trick, which isn't ideal and will be addressed in the next production run, is that you have to unscrew the ladder while the platform is open to slide the beading in that front channel.

It's a bit of a stretch to get all the beading in the channels, but we usually  start feeding the beading in to the sides, but do not pull the beading all the way towards the vehicle until  the front beading (the ladder side) is slid through.  Then you can pull the entire annex back a little bit at a time from each side.