After you have selected the button look at the properties of that button on the right side. In Entity select the entity you want to create. You could also use the Add widget on an empty space and select Create button. It will then open up another page where you can select the entity.
This looks like a common problem, Not clearly mentioned in the chapters.
I struggled a lil bit and found that, in Properties tab on right side, Under Events, you need to select “Create Object” for Onclick field. Then Entity starts appearing.