I'm having the same problem:
New installation on a Huffy Beach Cruiser.
I didn't get any instructions with my kit regarding the sprocket, so I went ahead and cut a slice through one rubber spacer to allow it to go inside the spokes and sandwiched the spokes. I faced the bolts in toward the hub and tightened in a star pattern (backing plates installed inside of course) watching the center to make sure it stayed in the middle as I went. (The sprocket doesn't drop onto the hub, it just sits on the end of it, but you can see that it's fairly well centered.) Once it started to move off center, as the bolts were getting tight, I loosened that bolt back up and tightened the three opposite it. At a certain point the sprocket would just go off center in the direction of the bolt I was tightening, I had to go in 1/4 turns in a star pattern from there to keep in lined up. And I still don't have it too tight because I got to a point where it wanted to shift and I couldn't adjust it back, so I left it a little loose. It's straight but I am considering taking it off and filing the center out enlarging it to fit the hub better. Meanwhile, to test fit everything, I put it all together (I haven't tried it out yet, just got it together today) But the problem is, the chain is rubbing on my wide beach cruiser tires, so it turns out, if I had filed the center, it would only make the sprocket closer to the tire and the chain rub worse. I think I can turn the sprocket around so the high side is outside once I file it to fit the hub and hopefully this will happen to be the right distance away from the frame so it doesn't rub, while leaving the correct amount of tire clearance. I am hoping there is a better way than hit and miss here but I suspect not. Pics attached -Scott