I have tried GitHub on a Mac with OSX 10.9 and it worked normally. I signed up to a branch and then perform a synchronization and all the files downloaded normally. If you have issues, try turning the Firewall off for the time being and download the latest version of the GitHub. On the other hand, it's possible that you've encountered a slow cluster to download the files from. Restart the software and then download again. It should be fixed.