As of Weka 3.1.8 access to visualization from the ClassifierPanel, ClusterPanel and AttributeSelection panel has moved from being a button to a popup menu. Click the right mouse button over an entry in the Result list to bring up the menu. You will be presented with options for viewing or saving the text output and---depending on the scheme---further options for visualizing errors, clusters, trees etc.
New in Weka 3.1.8 is the ability to visualize ROC (Reciever Operating Characteristic) curves for classifiers from the ClassifierPanel. Options for viewing these curves appear in the visualization popup menu.
New in Weka 3.1.8 is the ability to print how much memory is available in the Explorer and Experimenter and to run the garbage collector. Just right click over the Status area in the Explorer/Experimenter.
New in Weka 3.1.8 is the ability to save the currently viewed set of instances from the VisualizePanel.