There are a few options for filling your carto's. If you have a syringe that to me is the easiest way. You pop off the rubber plug, that's the end without the threads, fill the syringe with your juice of choice, insert it down the outside edge between the stuffing and metal wall, slowly fill at about a 45 degree angle, when it starts dripping out the threaded end it's full. Let it sit for about 10 minutes at least, blow through the non treaded end on to a paper towel, it'll knock any excess juice out and you are good to go.
Another way is to use the big rubber condom lookin caps that came on it. It is much easier to check out a YouTube video than have me explain it. Just search cartomizer condom fill.