If you're meaning the 24 gem Mystery Boxes:
When purchasing a Mystery Box, you are rewarded one random deco for your 24 gems. After your gems are deducted, you should receive a pop up that shows you which deco you received.
Example of Pop Up:
To find this deco, you will have to scroll through either the Floor Deco or Wall Deco tab, depending on which deco you received. Note to help in locating new items: the Mystery Box decos will be located toward the beginning of each tab, and will not have a price listed above them.
Example of what item looks like in Inventory:
If, after looking carefully through your inventories and you still cannot find them, try a reboot. If the items are still missing, then it's time to contact s8 Support.
This thread lists each Mystery Box and the possible prizes contained within each: https://forums.storm8.com/showthread...ster-Box-Guide
If you're meaning the 8 gem Boxes available during 15 day goals:
The Parts Crates for 8 gems do not gift items you can place in restaurant, it simply provides six random parts used to build the current goal appliances. These six parts rewarded will be a mix of the four new parts needed to build the two current goal appliances.
To see which specific part you were awarded: simply tap the Main Menu Button, then tap My Gifts, you will then see each and every part you have at your disposal to use whenever you like. On iOS devices, newest parts added to your inventory will be at the very beginning of the list. On Android devices, it's the opposite and the newest parts added to your inventory will be at the end of the list.
To use those specific parts you've won, you must participate in the current goals which will in turn unlock the two new appliances. Once you've unlocked the new appliances, you'll have to purchase them which will be a box of parts at first. Those new parts you've won will now be used to construct the appliances so that you will then be able to cook on them.
I hope that I've thought of everything to explain this process, if I didn't, feel free to post follow up questions. Happy gaming!