When I right-click the content and select add/remove tags the tag is listed, but it's not available in the filter.
The Filters Panel in the Tag & Filters Editor says "All" by default when you first create a Channel. It may be displaying "All", but still be updating as you are adding more tags with different keys. Try changing the drop-down to "1" and then back to "All", and see if that updates the Filters Panel.
It is the combo-box on the bottom right where it says "Active Keys". The key after the fourth row are grayed out, meaning they will be not visible/hidden to users in the Channel View.