Hey Janie, here is a video on how to fill a j tank. It's basically the same as the smoketech tanks, so hopefully this will help:
J Tank Filling - YouTube
I'm thinking maybe you have the carto too high up in the tank if you can't screw it on your ProVari once the tank is filled. I never put a tank over top a carto to fill once the carto is on my mod, because putting the tank together after normally pushes *some* juice in the carto and can cause the juice to flow over the top of the filler and down into the center tube and if there is enough pressure down into your mod. HIGHLY unlikely event with a ProVari, but it's always good to develop good habits early on.
It might help if you could take a picture for us of what the carto and tank look like all put together so we figure out why you can't get it screwed onto your ProVari. Unless you mean it's too slippery. In that case, I hold the the drip tip and use that to screw it onto my ProVari. You'll need a kind of tight tip though so that doesn't slip and slide around. One thing to note though, the carto doesn't have to be screwed on super tight, just tight enough to make contact with the flat pin in the center of the 510 connector on the ProVari.