The add on must be used with a spreadsheet. You can use it with an existing sheet but it would be better to create a new spreadsheet after you have installed it from the Workspace Marketplace.
To get started
click on Add-ons in the Main Menu
Select Classroom Teacher from the add-ons list
If Classroom Teacher is not listed scroll down to the bottom of the list to choose Get Add-ons and search for Classroom Teacher from the marketplace
From the Classroom Teacher menu choose Start/Setup - this will create some new tabs in the spreadsheet and open the sidebar menu
In the sidebar menu choose Classes and List - this will add a list of all your classes to the Classroom Classes tab