We will need some more information to be able to better provide any help to you. Is Express up to date? Currently the latest version is 3.2.25 see: How do I update Garmin Express?
Is your 310XT on the latest version of firmware? Current version should be 4.50.
To check your firmware:
1) Press Mode
2) Settings
3) About Forerunner
Have you ever been able to Sync? Does any of your data upload successfully? Is there ever any errors? Please provide as much information as you can to better try and determine what the cause of the problem is.
I am sorry that you are having trouble with Garmin Express. The latest version of Garmin Express is currently 3.2.25. If Garmin Express does not update itself, please make sure that there is nothing blocking your internet connection and you are not on a corporate or government network. You can also download the latest version here:
http://download.garmin.com/omt/express/B/GarminExpressInstaller.dmg
If you did a hard reset on your device, there also shouldn't be any activities remaining on it. There may be some files left over on your computer that could be causing the problem.
Please try these steps if you are still having trouble with the latest client:
Remove your device from Garmin Express (How do I delete a fitness device from Garmin Express?)
Quit Garmin Express from the black triangle icon on your menu bar
Delete these folders if either of them are still present:
~/Library/Application Support/Garmin/Devices/[Unit ID]/
~/Library/Application Support/Garmin/Express/RegisteredDevices/[Unit ID]/
