OpenCart is an opensource cart framework that is easy to use and manage.
Extensions
-> Extension Installer
and click the Upload File button to upload the previously downloaded zip file.
Extensions
-> Modifications
and hit the refresh modifications button
Dashboard
and click the settings icon shown below
Choose the extension type
and select Modules.
Catalog
-> Products
section in the admin and click to edit any of the products.print
, it should show Print.App drop option for you to select.