Scorestreaks must are 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.