You need a jvm of some kind which you can
get from here.
Since this is a swing based product, you also need
a version of swing. If your jdk is above 1.1.8 ( ie 1.2 or 1.3)
then you already have swing installed.
I don't think JSkeleton runs with a 1.1.8 VM anymore. You
will need jdk 1.3.