NetworkPrioritizer requires Java 1.6 or later and Cytoscape 2.8.x.
NetworkPrioritizer will soon be available for Cytoscape 3.x, as well.
To install NetworkPrioritizer, download the .jar file and save it into your Cytoscape plugins directory. After the next start of Cytoscape, you should see the NetworkPrioritizer menu entry in Cytoscape's Plugins menu.