Landing is like riding a bike, I can tell you how to do it but you have to figure it out yourself.
When you think about it, flying in from a few miles at 1000ft in a 152 to then land on a small line is pretty hard really. I was never spot on the centre line at 30 hours. Not even commercial pilots are spot on the centreline 100% of the time, so don't worry too much about being dead on it.
Something to remember is that the center line isn't directly down your eye sight. So for your nose gear to hit the centreline, you eyes sight will be to the left of the line. If your eyes match up with the centreline, then you are not on it and you are probably slightly to the right of the center line.
I don't really know what to suggest without being there and seeing you land myself.
Ask your instructor why he asked you to use right rudder and tell him you think you land on the center line and don't understand why he says you are not. He will explain it. Maybe it was just the wind or could be something else.