You cannot post in the forums with your admin account (unless anonymous posting is allowed, and even then you would be posting as guest and not an admin). Admin accounts are only for administration, if your create an admin and give privileges to the Forums it allows them to use the Forum admin, but does not give them any posting or moderation abilities.
You need to be logged in as a user to post in the forums. Just remember you can only post with "user" accounts and that should sort out some of the confusion.
You can also give individual users permissions in the forum admin.
http://webnet77.com/flash-tutorials/phpbb2-admin/phpbb2_admin_userpermission.htm
I think your permissions for the forums are too restrictive, are they set as I suggested in the other post?
If you are concerned about people registering and spamming, you can require admin approval for new users, or even turn off registration completely if you will be manually creating accounts.
yoursite.com/admin.php?op=yaUsersConfig
(under the registration tab)