I am using OS X 10.6.8. Somehow, they forgot to include this version in their AHT support page! Probably, it is the 2nd one that is applicable.
Using Apple Hardware Test on computers with OS X 10.7 or later
Using Apple Hardware Test on computers with OS X 10.5.5 to 10.6.7
Using Apple Hardware Test on computers with OS X 10.5.4 or earlier