You can't add content to tabber visually. You need to use source mode. If you are using visual mode, you can click the tabber and a box will pop-up with the option to edit them. It will then display the tabber content in source mode within the visual mode.
You can read this section about how to create tables in source mode. Alternatively, you can create the table outside the tabber using visual mode and then use source mode to move the table into the tabber.