mgrclient is a companion module for mgrnotificartion extra
(Jump to Properties Table.)
Go to System | Package Management on the main menu in the MODX Manager and click on the "Download Extras" button. That will take you to the Revolution Repository (AKA Web Transport Facility). Put mgrclient in the search box and press Enter. Click on the "Download" button, and once the package is downloaded, click on the "Back to Package Manager" button. That should bring you back to your Package Management grid. Click on the "Install" button next to mgrclient in the grid. The mgrclient package should now be installed.