I too was unable to use my Mappillary account for the forum. No matter what I did I'd come back to the statement that there is no Mappillary account under my email, even though my email is posted on my. Profile page.
The first time I created a new account and was able to post, when I came back to the forum an hour later I was told my account did not exsist (sp)
This forum account is using my 3rd email.
I agree that a hint might help. In any event, the creation and maintaining of a forum account would benefit from being made easier.