There are some common reasons why you may not be able to view an application.
The tool is internally misconfigured in nanoHUB. If you can look at your my nanoHUB page and see that the tool you just started does not appear in the list of running sessions, it exited before it could be viewed. You should report this as a problem.
Your Java interpreter won't start or is misconfigured. If you've been able to view applications in previous times with the same browser, usually you can solve this problem by restarting your browser. Make sure that all instances of the browser (e.g. multiple windows) are stopped in order to properly stop and restart the Java environment. If this fails to make tool sessions viewable, consult the Java page for more platform-specific hints.
You can test whether Java is working correctly in your browser by going to the generic applet testing page on java.com. You can also find more information on troubleshooting Java problems on that page.