So first as explained in the article I send previously go to "Tracking and Analytics" (sorry my wix is in french)
Then select "Custom code" and there you must be able to paste the code form Naker.Back editor!
Next enter in "Developer mode"
So that you can see and set the id of the div which will receive the back:
I hope this is more clear for you. Tell me if there is something missing! 😉