Somehow the default Iced Tea Webstart throws NullPointerException which I dont know how to debug. I downloaded SUN JDK and installed on my machine, you can use your package manager also. It usually gets installed in /usr/java/jdk-<version> and /usr/java/latest links to the latest version installed, so it is better to use the later. Then download the ContestApplet.jnlp and simply sun the following command to get it working.
$ /usr/java/latest/bin/javaws ContestApplet.jnlp
It uses default system proxy to download the content from Topcoder needed to execute the arena. When the arena is open set the proxy and use HTTP Tunnel A or B. For us Tunnel B works.