if it doesn't work ... despite having he correct line in your ~/.ssh/config :

Host foobar.pizza
  ForwardAgent yes

... and that your global configuration does not override your local configuration (check /etc/ssh_config) :

 Host *
   SendEnv LANG LC_*
#  ForwardAgent no

Well, on OS X, You just need to (Mac OS X 10.8) :

ssh-add

And voila!