Why does git:// works
$ git clone git://github.com/schacon/grit.git
Cloning into \'grit\'...
...
Checking connectivity... done.
but git@ do
Your first clone method is using the git protocol, the second is using SSH.
You probably don't have your SSH token setup on github.com
https://help.github.com/articles/generating-ssh-keys
Gives you the steps on how to setup your user account for SSH use.
You can see the differences between the protocols as they pertain to github here:
https://gist.github.com/grawity/4392747
This is because git@ uses ssh protocol. It is equivalent to ssh://git@.. So if you dont have correct ssh keys it will not work. Option git:// however uses git protocol which is similar to ssh but uses no authentication at all. See chapter on protocols for more information.