How did you obtain CS5? If you got it at the Adobe online store, then go back and the info will no doubt be there. If you bought a DVD, then I would expect instructions to be on the case, or on a read me file on the DVD.
That sounds like an online store. If so, have you tried logging on to your account, and checking 'My orders'? I am not sure how the free upgrade would work, and I have a feeling that trying to phone Adobe is going to be a long and frustrating experience.
There are quite a few Google hits HERE