Go into the admin workspace, external tools and then add a tool:
Select the link titled "Allow the Tool to provide a common cartridge"
Go into Mercury->Lessons and click on "More Tools" and not that the newly installed tool does not appear. Go back into the external tool and tick the "Provides a file" and resave the tool.
Go back into lessons and "More Tools" and the tool will appear.
The wrong checkbox is controlling this behavior.