I also very much recommend turning the Mac upside-down. I struggled to get the pegs in the holes, until I held the Mac upside down and started wiggling the drive. It was pretty easy to see when the drive was seated correctly: there's a plastic tab under the top drive. If the pegs are not in the holes, you'll see that the drive is flush with the plastic tab. When the pegs are in the holes, you'll have around 2mm clearance between tab and drive.