AHT problem: how to run AHT on imac os x lion ?

imac june 2010 and it came with snow leopard. i did upgrade to lion online 2 years ago and since yesterday litttle colour/ black squares appeared on the screen which after one restart spread all over and i can not see anything on it at all( refer to the attached imaga)

i am trying to run AHT and find out the problem. the dvd which came with imac is snow leopard and it doesnt work! also i can not see anything on the screen due to this squeares

could someone please guide me how to run AHT.i did try to tun AHT online and it says i can not.