Comment 7 for bug 119175

This could help you:

http://dmartin.org/weblog/running-java-web-start-apps-on-ubuntu-linux-for-amd-64

"The solution is to install the 32 bit Java 1.5. I realize Java 1.6 is available, but it won't work for some reason (I couldn't really tell why, the app just never comes up, but shows no errors). The installation is pretty simple."