OK... I had this problem with my 2012 Mini 6,2. I fixed it "my way."
First of all, there's no "groove" for the curved part of the antenna to fit into. It fits UNDER the lip of the main case. You can see no groove would work, as part of the curved antenna edge is clamped to hold the antenna wire. It is too wide to fit a groove.
Another problem is that the little screws only have about two threads to hold those things. I'll bet Apple saved about a tenth of a cent per unit by saving on screw length. I guess for Steve Jobs, "length mattered."
First, I removed the antenna and its wire. I thought I'd broken the wire off the connector, so I took that as a warning. Who wants to solder such a small connector?
I used a small leather awl on a multi-blade knife to elongate one hole on the antenna into a slot. I then inserted the antenna as best I could, slightly turned one screw to hold it, then aligned the 2nd, elongated hole and jiggled the antenna until the screw turned. I still had to use a "hook" as described above, to bend and unbend the antenna so its length would be correct.
This still made a 45 minute job take about 2 hours.
Ironic, since I'm doing this so I'll have a 2nd drive to install Linux as a dual-boot. Eventually, OSX is going bye-bye. I'm sick of the bugs, and won't "upgrade" beyond Mtn Lion, to a version that requires me to let the "cloud" suck stuff off my computer! Knowingly, anyway.
Next upgrade will be to an all-Linux, generic PC I can configure, upgrade and work on without feeling like I'm doing an eye transplant on a hummingbird.
Hope this helps someone.