Yes, adding todos should be possible.

Steps to do to add a todo (sorry for the word game...): tap on the "plus", type a title for todo, swipe the calendar left/right to pick the desidered one, tap on "Add". Calendar is the same of the picked one in the main page, it must not be "All" or you can't add a todo.

If a todo is checked in default calendar app, is it checked as well in TodoLister?

Thanks for the help.