What finally helped me was to check things one at a time. To check the tongue-and-groove fit, I installed the antenna plate by itself, with the hard drive removed. I used a spudger to pry open the groove on the antenna plate, since I had closed it a bit by forcing things. After the antenna plate was going in by itself, I focussed on the hard drive. At first, I was pre-attaching the hard drive to the antenna plate, but this makes it hard to see if the hard drive is going in as far as it should. Getting the two protruding screws on the hard drive into their holes is tricky (I found turning the Mini upside down to use gravity helped), but for me it was some wires at the side of the hard drive space that were getting under the drive and making it sit up high that was the problem. Once I got the hard drive in, and then the antenna plate, I could get the last two screws which hold the drive to the plate to catch by turning the Mini upside down and tapping it on top to bring the hard drive close to the plate.
What finally helped me was to check things one at a time. To check the tongue-and-groove fit, I installed the antenna plate by itself, with the hard drive removed. I used a spudger to pry open the groove on the antenna plate, since I had closed it a bit by forcing things. After the antenna plate was going in by itself, I focussed on the hard drive. At first, I was pre-attaching the hard drive to the antenna plate, but this makes it hard to see if the hard drive is going in as far as it should. Getting the two protruding screws on the hard drive into their holes is tricky (I found turning the Mini upside down to use gravity helped), but for me it was some wires at the side of the hard drive space that were getting under the drive and making it sit up high that was the problem. Once I got the hard drive in, and then the antenna plate, I could get the last two screws which hold the drive to the plate to catch by turning the Mini upside down and tapping it on top to bring the hard drive close to the plate.