Calculation of dunn index

Hi,

http://www.mathworks.com/matlabcentral/fileexchange/27859-dunns-index/content/dunns.m

In this link calcualtion of dunn index has been given but i get error as index exceeds dimension in following line

temp=distM(x,y);

I would like to know how this distM (dissimilarity matrix) should be represented

and even how to calculate inter cluster distance

Regards,

Amishi