Use the 'Labels' property of the dendogram plot. Keep in mind that dendrogram labels any leaves in the dendrogram plot containing a single data point with that data point's label.
For example, assume you have a dendogram with 5 leaves, create a cell that lists {'label 1', 'label 2',...,'label 5'}, and set this cell as argument of 'Labels' property. This will assign 'label 1' to leaf 1, ..., 'label n' to leaf n.
X = rand(5,2);
Y = pdist(X);
Z = linkage(Y,'average');
%create cell of labels
labels = cellstr(num2str((1:size(X,1))', 'label %d'))
%plot dendogram with custom labels
h=dendrogram(Z, 0, 'Labels', labels, 'orientation', 'left');
