Recently had to complete this repair for a customer. Was in the same boat, couldn’t figure out how to plug the battery back in. And wasn’t paying attention to how the connection was originally, when i pulled the battery. The connector has to slide UNDER the teeth, not above. Meaning loosen the board screws and lift the board slightly, then slide the battery connector in. hopefully this helps others that a stuck with this issue, and iFixit will update this guide.