There are no known issues with Garmin Express 4.1.7 recognizing ANT devices in 10.10.5. I couldn't find any information on you in our system. Have you had the Forerunner 910XT paired to Garmin Express in the past? The fact that you mention it charging but not being recognized leads to me to mention that the Forerunner 910XT doesn't communicate through the USB cable but rather it communicates wirelessly through the Garmin USB ANT Stick. Insert the ANT Stick in the USB port then enable pairing on the watch and add it to Garmin Express.
To enable pairing on the Forerunner 310XT or 910XT:
Navigate to Main menu
Select Settings
Select System
Select Data Transfer
Change Pairing to On
Forgive me if this is rudimentary information but it does happen. I've also included a FAQ below for more general troubleshooting.