hi i came across this problem that while trying to launch any applet or run a .jnpl file it failed to load. I checked my system and found that java was installed but not sun java ,it was icedtea . So i found a solution to it in ubuntu.
Go to System->Administration->Software Resources->Other Resources and enable to canonical repos.
Next just install sun-javaX-plugin. (where X is the latest version).
Hope it solves your problem.
Let the source be open :)