Okay... so I got my starter kit with two batteries, two atomizers, some blank cartridges, some pre-filled cartridges, and some juice. Problem is... the atomizer seems to already have some kind of cap on it, and I can't for the life of me remove the cap so I can put a cartridge on it. I'm twisting and pulling and it just won't come off. Am I even supposed to take it off? How can I put the cartridge on?