Um, no, you can't. Siri only is on the iPhone 4S, at the moment.
If you're jailbroken and you've installed Siri0us from Cydia, be aware that the app’s developer withdrew his app due to licensing issues. Evidently, he was illegally using Nuance software (Nuance are the folk who invented Siri/the voice recognition). Nuance has told the developer that it won’t let him buy the rights to use it lawfully.
So, if you have it, then it's just a matter of time before it'll be unusable. And, if it's not working now, it's not going to. IOW, we can't help you, because it's an unsupported hack. And, honestly ... I don't see it becoming a viable hack, what with Nuance not wanting to jeopardize it's relationship with Apple, the iPhone 4S and Siri.
Sorry - it's just a no-go.
Marilyn