I'm not certain I fully understand your question, but I'll make a guess.
The centroid is not (necessarily) one of the points of the original dataset, so none of the rows of idx correspond to the centroid itself.
The centroid locations are given by the rows of C (not the columns). The row C(1,:) is the centroid for the cluster of points with idx=1. The row C(2,:) is the centroid for the cluster of points with idx=2. And so on.
If that doesn't answer your question, maybe you could comment with some clarification.