If you imported the demo content successfully, you should have imported the necessary widgets. Just go to Appearance > Widgets and verify you have the "Open Table Widget". This will likely appear in the "Inactive Widgets" list on your Widgets page. If not, that's ok, you should see the Open Table Widget listed in the Available Widgets area.
Simply create a new widget area called "Reservations" (or name it whatever you would like) and drag/drop the Open Table Widget to this section. You can open up the widget to define your restaurant name, Open Table Restaurant ID and any other options you'd like to change. Hit the save button when complete.
The final step is going to your Reservations page and opening up the Sidebar widget and choosing your newly created widget area.