This is a completely different add-on from the original Morning Coffee, so it won't know about the sites you set up with the original. You'll have to set them up again. The best way to do that is with the tab context menu, as illustrated in the screenshots. Right-click on the tab for a site you want to add, select the "Add Tab to..." menu item at the bottom of the context menu, and select the day or days you want. Once you've done that, the button should work as you expect.

If this works for you, please consider revising your rating. If it doesn't, please send me an email and I will do my best to make it work. Thanks for trying it out!