Hi there,
I think I know the problem. When you first came to the petfinder page you had to log-in to join. When you joined petfinder I believe the user ID is your email address and then you pick a password.
In order to reply to threads and post messages you need to log into the forum section. THIS IS A SEPARATE LOG IN! You pick your id/name and password. When you click on the forum section I believe the log in box is in blue? and to the right of the page. I'm guessing that you were trying to use the SAME log-in for the forum as you did for the home petfinder page and if you hadn't registered in the forum section it won't work.
Again, not 100% sure this is the problem you encountered but think it might be.
Keeping my paws crossed that the above info helps!
Woof,
-Mutt