To use stuff from the store, if its an attack item you have to attack the person the right way to use it. Say it's an inteligence offense item, you have to dance off somebody to use it. The same for fights with strength items.
For defense items they get used every time you get attacked. So if you have a str defense item, then when you get hit one will be used. Again, same for dance offs, but with inteligence items.