You can find your item code to redeem, either in the window you are presented when first purchasing the item, or by navigating to "My Prizes"
For rewards from missions, you will also find these under "My Prizes"
To redeem your code, launch Battalion 1944, and navigate to the "Armoury"
Select your "Inventory"
Once in the inventory, you can redeem your item code by selecting "redeem code" at the bottom of the screen.
Enter the code and hit submit and you will receive your item. Please note, you may need to leave the armoury and then return to it to see your new item.