I have a very large amount of data in the form of matrix.I have already clustered it using k-means clustering in MATLAB R2013a. I want the exact coordinates of the centroid
The centroid is simply evaluated as the average value of all the points' coordinates that are assigned to that cluster.
If you have the assignments {point;cluster} you can easily evaluate the centroid: let's say you have a given cluster with n points assigned to it and these points are a1,a2,...,an. You can evaluate the centroid for such cluster by using:
centroid=(a1+a2+...+an)/n
Obviously you can run this process in a loop, depending on how your data structure (i.e. the assignment point/centroid) is organized.