You can find any boosters you receive in the Booster Menu, a star that appears to the left of your main menu bar at the top of the screen:
However, this doesn't appear until you reach level 10. If you receive a booster because you logged out and back in before then, or redeemed your starter pack, you won't be able to see it until later (but don't worry - it will be there, waiting for you, at level 10!).