Your tag databases are listed in the main Table of Contents in the Databases section, directly below Tag Index. To see an index of all the tags in a database, click on its entry in the Table of Contents. The Tag Database index is a list of all the tags referenced in that database. Clicking on any tag will then bring up a description of that tag as listed in the database.
Selecting this tag database file with the index pane in Treeview would look like this: