How to run the Apple Hardware Test in Snow Leopard on a MacBook Pro? I have a MacBook Pro with Snow Leopard.

I have an image of the Installation Disk in a USB Hard Drive (Snow Leopard Partition). Then to boot the system and start using the Hardware Test software I did this:


  
  I change the Starup Disk to the 'Snow Leopard Partition'
  I restart holding down the 'D' key 
  


But it shows me a gray screen and the it start from the Snow Leopard Partition. But I can't see the Apple Hardware Test software.

What am I missing here?
If I remember correctly, in Snow Leopard the Apple Hardware test was on the second install DVD, and would only run from the DVD (unless you manage to install it on the hard drive and run it from there).

See also the official Apple page: Apple hardware test