By your written description, I think I know your problem. The wide/chamfered part of that nut goes UNDER the sheetmetal bracket, not on top of the bracket. Only the cylindrical and hex-shaped part of the nut should be on top, this part "pokes through" that kind of key-hole shaped hole in the bracket. Look at that image that was first posted.
You need to thread the nut onto the bolt, put the collar onto the tensioner (rod goes towards the engine side), and put this whole mess up through the bracket. One tip is to thread the nut down so the tip of the rod is about even with the top of the nut 9up even with the top of the hex part).
Hope this helps

Gilly

ps Look at the bottom part of the nut, it's UNDER the bracket:
http://www.peachparts.com/shopforum/attachment.php?postid=482113