Is Cytoscape 3 installed? If not, download it from here: http://www.cytoscape.org/download.html

Is Cytoscape running? If not, go ahead and start it.

After Cytoscape has started, switch back to the App Store and reload the page.

Navigate to the app you want. Now click Install.

Wait for it...

Okay! Your app is now installed. Switch back to Cytoscape and start using it.