The ToDo list function isn't working on entities. If you click on the To Do list tab on an entity, the edit button is greyed out, and you are only able to change the name.
Need to save the ToDo item first.
After you added a ToDo item, you need to save it. In the To Do screen, click on the Apply button and then OK.
Now go back to To Do again and you should see the Edit button enabled.