'In Administration Control Panel (ACP) > FORUMS > Manage forums make a new forum.
Forum type: Category
Parent forum: No parent
Set permissions for category.
Then make another forum.
Forum type: Forum
Parent forum: the category you just made.
Set permissions for forum.
To set permissions, either use the option to copy permissions from an existing category or forum that has the desired permissions, or: Administration Control Panel (ACP) > Permissions > Group forum permissions
Set the registered users to be able to see and post in it: Registered users > Standard access
You can trace the problem with the permission masks
Bottom of the permissions menu is the masks section
If you choose forum based permissions and then on the next screen select that forum/category that you know you should be able to see and then enter your username in the bottom left box, on the next screen you'll get the permissions for that forum.
On the left of the line for 'can see forum' you can click the O-> icon and it will show you the groups you're in and where the blocked permission is coming from.