In the US, you can only get the prestige token if you pre-order from Gamestop.
I found these instructions at http://callofduty.com forum:
"Your receipt should have 2 codes on it. One you register on the gamestop site http://gamestopblackops2.com/ and the other you go to http://callofduty.com/prestigetoken. Sign in and put the code on that site. The token code is the one with the dashes in it. The Gamestop one to register your preorder for 3 other waves of bonuses is just numbers/letters."
Here is a picture of where you'll find the promo code from Gamestop:
If you ordered from the UK, here are instructions I found for that:
Terms and Conditions: UK ONLY. Limited Offer while stocks last. Customers who have already preordered Call of Duty: Black Ops 2 will also receive the Prestige token. You will need a copy of Call of Duty: Modern Warfare 3 in order to use your Prestige token. You will need access to PlayStation Network (PlayStation 3 Customers) and Xbox LIVE GOLD membership (Xbox 360 Customers) in order to access your Prestige Token (If you don't have Xbox LIVE GOLD membership you can buy it now). You will also need a unique Xbox LIVE Gamertag or PlayStation Network ID. In order to receive your Prestige token you will need to login/create an account on http://www.callofduty.com/prestigetoken and enter your access code. Access codes for http://www.callofduty.com/prestigetoken will be emailed on a rolling weekly basis starting Thursday 3rd May. The last chance to preorder each week for the next batch is Wednesday at 11pm. Please ensure that the email address in Your Account is valid before preordering. Please allow 48 hours and check your junk mail before contacting Customer Services about your access code. Once you have entered your access code at http://www.callofduty.com/prestigetoken you will then be emailed your token by Activision. Activision reserve the right to change the terms of this promotion in the event of any change of circumstances outside of their control. The access code and Prestige token have no cash value and are non-transferable. This offer is only valid against mint boxed copy of the game and not the download or preowned version. Bulk or Business preorders (orders in excess of 10 copies of the same game) will only receive 1x bonus code per order not per copy of the game.