When playing multiplayer in COD: MW3 on the PC you are able to select strike packages when creating a class. How do you execute them during multiplayer?
The "support" and "assault" strike packages will give you rewards depending on how you set up your loadout, and how well you're doing in the current match.
When you've earned a reward, you'll get a notification on your HUD, and pressing a key will activate it. This key is configurable, but by default I believe it is 4.
The "specialist" strike package grants additional perks as you get kills without dying, but this does not require any sort of activation.