As the manual link states, checkboxes only work in plain lists. Your example uses headlines. You need to use the checkboxes this way:
* shopping list
- eggs
- bread
- Tesla Model S
- bacon
Now, when you're on "eggs", you can hit C-u C-c C-c (C-u C-c C-x C-b also works) to toggle the checkbox. C-c C-c will then toggle whether the checkbox is checked or not.
If you are already in a checkbox list, then M-S-RET (or Alt-Shift Enter) will add a new item with a checkbox.
