You need to pierce the "membrane" in the center tube that you mentioned. Membrane is used pretty loosely in my mind because it takes a good bit of force to get through it. I've had the best luck using a large safety pin heated a little to help melt through the plastic. Makes a nice hole that is large enough to take that 18 ga blunt needle. Then you attach the blunt 18 ga to a 3 - 5 ml syringe and add your
juice through that hole you just made. I find at best I can get 1.75 - 1.9 ml into a new one. It's a good idea to go very slowly to let the
juice soak in well. You could also spin the carto (Taryn spin??? not sure of the name) to help push the juice into the filling. If you push the juice too fast, it will just come out the end hole or come up through the half circle you saw. You certainly can't push the fluid quickly like you would on an IV port. Have to go slow.... maybe if the juice is of low viscosity, you could go a little faster..... all my juice is pretty thick, so I can't say for sure. But that is the general "drift" of how to fill it.
I really like the vapor you get from it. Once in a while right after filling I'll hear a little gurgling. Probably too much juice.
When it comes to refilling, I've never quite figured out how to know exactly when to do so. I don't want to run it so dry that I might burn the filling and trash it, so I generally will add say .5 - .75 ml after a couple of hours of use. I'm probably being way to conservative, but I'd rather keep it wet in there than run it dry and ruin it. Hope this helped a little........