sudo a graphical app? still not working

[kellyremo]

Fedora 14:

i cleared all the firewall rules, then:

visudo:
Defaults    env_keep += "DISPLAY XAUTHORITY"
MYTRUEUSER  ALL=(transmission-user) NOPASSWD: /usr/bin/transmission-gtk


$ sudo -u transmission-user -- /usr/bin/transmission-gtk
No protocol specified
Cannot open display: 
$ 

if i ps | grep transmission, i can see it's running.
how could i see the transmission's GUI? 

i tried:
xhost +127.0.0.1

but it doesn't work, error message is the same.

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