In order to authenticate with the print server, the PC Client Application must be running.
To open the PC client Application, click the icon on your Dock that looks like this: 
It can also be found in your Applications folder, or by pressing Command + Space Bar at the same time, and searching for PCClient.
When running, the application will be displayed with the White 'P' icon on your Menu Bar, up by the date, time, and wifi connection.
Once the application is opened, and a print job is sent from the computer, the Application will pop-up an Authentication screen, asking for your Username and Password.
Username & Passwords are connected with your Google account. Eg - Firstname.Lastname. (you do not need @msad60.org at the end)
Password = your Google Account Password