Please note that some items (included in your Pixel Pack) are available in the Battle HQ only after reaching Chapter 3 of Ancient Garden.
To find your unlocked items, head to the Battle HQ. You can find it by taking the path to the left of Peach Castle. It is located in the first building you come to.
If you redeem a code for the Steampunk Pack for example, you will be able to find the items of the pack in the Weapon Selection menu of the Battle HQ.