Are you doing this from a remote system?
For clarity, lets say the box that you want to export X from is named A and the one you want it to go to is named B.
You cant do
on A and expect X to magically appear on B. There has to be an X server listening on B.
If B = Windows, you can use Exceed or any of a hundred others. If B = Linux, you can just SSH to A and run xclock & and it will appear on the desktop of B.