First, to wear anything from your inventory, you can right click it and select wear....right click the folder it's in and select add to outfit, or....if it's a complete avatar, right click the folder and select replace outfit.
If you try to wear something, and find yourself wearing a box, DON'T PANIC!....We all wear boxes at one time or another. Simply get to a sandbox or some place where you can left click the box and drag it to the ground. Once it's there, right click and open the box. You can then either elect to wear the items in the box and copy to inventory, or just copy to inventory.
Hope this helps.