Make sure you installed the right version of Cytoscape (2.6.3). Also, if CommFinder was installed manually, make sure the jar file is placed under the plugins directory.
Delete temporary files, refresh the webpage and try again..
Issue "chmod +x MCRInstaller.bin" command, then try to run it again.
Disable the visual settings: System => Preference => Appearance => Visual Effects => None.
Modify the default web browser setting on Cytoscape to your current web browser: Edit => Preferences => Properties => defaultWebBrowser
The downloaded file might be corrupted. Try refreshing the browser and download again. The jar file size should be around 950 KB.
Make sure you have the path set correctly as specified on the manual.
Make sure the previous result of the algorithm is closed. You can export and save the previous result if needed.