In the main panel, under "Test options" click the "Supplied test set" radio button, and then click the "Set..." button. This will pop up a window which allows you to open the file containing test instances, as in Figures 27-a and 27-b.
Figure 27-a Figure 27-b
In this case, we open the file "bank-new.arff" and upon returning to the main window, we click the "start" button. This, once again generates the models from our training data, but this time it applies the model to the new unclassified instances in the "bank-new.arff" file in order to predict the value of "pep" attribute. The result is depicted in Figure 28. Note that the summary of the results in the right panel does not show any statistics. This is because in our test instances the value of the class attribute ("pep") was left as "?", thus WEKA has no actual values to which it can compare the predicted values of new instances.
Figure 28