The best way I found was to insert the spring into the tube, with the barrel pointed towards the ceiling, place the extension (making sure the end piece is in place) and slowly compress the spring and thread the extension on. It took me a couple tries at first, but it gets easier after the first couple times!
If it still doesn't want to go on, I would suggest calling Benelli to find out if you have the right spring. 33" sounds about right though.