You can use it in battle by going to the inventory. You'll have to play the cartridge again after that
I understand, I played the cartridge, went to the Employee Locker Room and grabbed the user Retriaval Device and place it in the Teleportation Chamber. Then, I went to User Locker Room and it asks me for a user. From there I couldn't do anything else.
Sorry if I don't express myself well, I'm from Uruguay xD