go into setting make sure its turned on and hold home button own for 2 sec
longpress on the home button. You can also bring it up to your face like you're going to make a call and it will enable it.
I saw a video review and the review said that Siri shipped not turned on. Check in settings and maybe general to make sure it is on.
not in settings??? not sure then
Settings-General-Siri
cool