If you have only bought Complete you do not get HttT or DW, and so 5.2 is of no use to you. Despite the name Complete is not complete - it was at the time it was released but they then changed their minds and released those two more expansions. As there were already boxes out there with Complete written on them they couldn't change its name either.
So you need to buy those two expansions, from wherever you bought your game, and register them here, then you'll be able to see the 5.2 forum.
I bought Chronicles so I have Divine Wind 5.1; I've never played anything HttT or earlier
I got it from the PC Gamer deal, so I can screenshot the codes, but I'm not sure I got a receipt.
EDIT: Just to make it clear, I have the codes for HttT and DW, but on the My Games page, there is no EU3: HttT or EU3: DW on the list to register.