Got my Riva 510 in today and I also ordered some cartomizers, along with the carts, just to see what I prefer, but I can't figure out how to put juice in the carto? I'm used to a KR808 D-1 carto and that was easy to refill but this new carto has a plastic piece on the draw end, which I took off, then there's another plastic piece down in there that seems to be attached to the wall of the carto?? I have no idea how to put juice in this thing!!