I am running Express 4.0.18 for Mac and my OS is 10.10.3. The Ant stick is being detected correctly. What OS are you running and what version of Express are you using? I know you stated the latest version I just wanted to make sure we are running the same version of Express. Now for the ANT stick can you please click:
Apple Icon upper left
About This Mac
System Report
USB
Should see ANT USB Stick if it's being detected
If the ANT stick is being seen and Express is indicated it's not being detected I would try completely uninstalling Express and reinstalling it. See FAQ: How do I completely uninstall Garmin Express?