Add row to class diagram

Select one of the existing rows and duplicate (ctrl/cmd-d or right click, select duplicate).

If you want to add a new row below the selected one use Ctrl+Enter

It's better solution if you are dealing with classes with method and attribute fields. (Ex.: Class figure)

If you have deleted all of the rows from the class diagram:

Then you will have to make a new class diagram and drag one of the fields into the old one:

Resulting in:

Then to add further cells you can follow the other answers.

As @Amadeusz and @RyanZim said, pressing Ctrl + Enter is a proper way to add a new row ON Windows.

ON macOS, you can press ⌘ + ⏎ to do that.