To represent the cluster assignments Weka adds a new attribute Cluster and includes its corresponding values at the end of each data line. Note that all other attributes are shown, including the ignored ones (play, in this case). Also, only the leaf clusters are shown. Now,to compute the classes to clusters error in, say, cluster 3 we look at the corresponding data rows in the ARFF file and get the distribution of the class variable: {no, no, yes}. This means that the majority class is no and the error is 1/3.
If we want to compute the error not only for leaf clusters, we need to look at the clustering structure (the Visualize tree option helps here) and determine how the leaf clusters are combined in other clusters at higher levels of the hierarchy. For example, at the top level we have two clusters - 1 and 5. We can get the class distribution of 5 directly from the data (because 5 is a leaf) - 3 yes's and 3 no's. While for cluster 1 we need its subclusters - 2, 3 and 4. Summing up the class values we get 6 yes's and 2 no's. Finally, the majority in cluster 1 is yes and in cluster 5 is no (could be yes too) and the error (for the top level partitioning in two clusters) is 5/14.
Weka provides another approach to see the instances belonging to each cluster. When you visualize the clustering tree, you can click on a node and then see the visualization of the instances falling into the corresponding cluster (i.e. into the leafs of the subtree). This is a very useful feature, however if you ignore an attribute (as we did with "play" in the experiments above) it does not show in the visualization.