Open the Notes Preferences dialog by choosing the menu option File – Preferences. You should see an option on the left for "panagenda Tabzilla":
If this preference page doesn't exist, you don't have the plugin installed. If the plugin is installed, you can see the version of the plugin at the bottom of the preference page.
Also, the Tabzilla plugin will always leave at least one tab open, to ensure proper functioning of the Notes client.
Tabzilla menu commands stopped working after I switched IDs
This was fixed in Tabzilla version 1.0.1. Please see Downloading the Tabzilla Plugin for information on how to get the latest version of Tabzilla.