And if you're trying to make a comment, either tap the comment box or type your content and tap send (I can't remember which since I did this only once so long ago), then you'll be prompted to log in. Tap ok to log in.
I believe the forums.wpcentral.com allows you to log in with other accounts, such as a windowsphone.com/live.com account. Is there any way to do this with the app, or at least link a WPC and windowsphone account in some way?
Try to comment or post on a forum. You will get a message saying you're not logged in. Tap "ok" and you will be taken to the login window. Hope that helps.