I unlocked a whole bunch of scorestreaks in the past few days, but I have no clue on how to use them. Is there a certain objective you have to accomplish in order to use them, or can they be activated with the press of a certain button? I'm on PS3 by the way.
Scorestreaks must be unlocked and selected out at the root menu of the multiplayer interface, shown here:
Note that once you enter that menu, you will see all of the Scorestreaks available for your level - but you may still need to spend tokens to unlock Scorestreaks which you have not previously unlocked, in order to make them "selectable". This is straightforward, and there are on screen indicators of Scorestreaks that require you to spend unlock tokens.
A few other possibly relevant details: