I am having trouble with this too. I used my Microsoft account to sign up to WPCentral forums, and can sign in via the web with no trouble, but in the App it asks for a User Name and Password, which I have tried what my user name is and the password for my Microsoft account, but it fails. Have tried all variations but nothing. When I then opt to Register via the app and sign in with my Microsoft account it tells me the browser can't run Java Script and fails to load the page.
I am thinking I would need to register as an actual user to get this to work, but then this negates what I already have done via my Microsoft account.