I just set up my 920xt using Garmin Express 3.2.25.0 and OS X 10.10.1. My Edge 1000 syncs fine with my Apple Airport base station, but I kept getting either Network couldn't be found or network error when trying to connect with the 920xt. I tried WPA, WPA2, WPA/WPA2, and played with different 2.4ghz channels. I also tried the iPhone hotspot and the wireless on the Verizon Fios router with no success. I finally decided to grab the Edge 1000 delete it's wifi preferences and see if it could establish the network and it couldn't. I got my Edge 1000 end of October, so I think I was on 10.10, but may have been on 10.9.x. So the only change I can think of was the update to 10.10.1.
So to make a long story even longer, I used Parallels and downloaded Garmin Express for Windows 8.1. I added my Airport router through that and the 920xt found it on the 1st try.