How to access apple support forum on "This username ... is already taken, please choose another. Another has been provided" message? See screenshot: This username "***" is already taken, please choose another. Another has been provided.

If I enter a different forum username it tells me that the account is already associated with another user ID (I suspect the first one I tried).  

How am I supposed to continue?
Go to Apple ID support web page at http://www.apple.com/support/appleid/ and use the contact support button.

Your account is in a state where the tool can't let you continue and you may need a human on the other end to know what exactly to do. A search of the public knowledge base shows nothing that you should do in this case.