Mobile Nations Passport isn't yet supported in the WPCentral app (but I've requested it in the official request thread here The Feature Request Thread)
Hopefully, it'll get sorted at some point soon (I'm in the same position). The only way around it at present is to edit your forum account (settings cog icon at top right of the forum screen) and change your password there. You'll then be able to log in to the WPCentral app and leave comments.
But using that workaround will lose the benefit of signing in via your MS account as you'll have to enter your user/pass.
Having said that, it's not a huge hassle to do so.