Sorry for raising this question here, but I am not getting any useful responses on the official D3 technical support forums.
I have an expired WoW account, but an active D3 account, yet I cannot post on the D3 forums, I get a message saying that my (WoW) account has expired.
In the pull-down menu in the upper right corner where I choose which character I am posting as, I see only a list of my old WoW characters, but no option to switch to a D3 posting account. Also, when I click on the "Profile" button right under my name, I am redirected to a page that says "OOPS" in big letters.
Problem solved, my D3 account was infact listed in the "manage characters" menu, it's just that my BattleTag name (under which the d3 account appears in the menu) and my old WoW main is the same, even the same colour, that is what was causing the confusion.