If you see the icon of the computer connected to the Edge on the device then there may be an issue with a file on your Garmin. Plug your device to the computer follow these steps:
Open the Garmin device (either myComputer/Computer or Finder)
Open the Garmin Folder
Delete the file GarminDevice.xml (maybe shown as GarminDevice)
Empty your Trash Can or Recycle Bin
Disconnect the device
Allow device to boot to main screen and leave it for 30 seconds
Plug back to the computer and see if Express sees it.
It sounds like the Mac itself is having trouble seeing your device. If this is the case please see the FAQ:
Why is my Garmin USB device not recognized by my Mac?