This was the hardest part of the repair for me. The iSight cable was really lodged in its socket. "Walking" the connector as the guide suggests -- i.e. pushing on each side alternately at a 45 degree angle (with more force than I expected) -- eventually worked.
As two other users have mentioned, the long-and short-sleeved cables for my late 2012/early 2013 A1425 were not in the order mentioned in the guide. If the laptop is positioned with fan in the upper left corner, the order (left to right) is short sleeve, long sleeve, no sleeve. If it is positioned as it is in the photo with the fan in the lower right, the order (left to right) is no sleeve, long sleeve, short sleeve.