After installing http://wiki.jenkins-ci.org/display/JENKINS/VirtualBox+Plugin I can create a cloud fine.

However when I create a node no matter what option I select for "Secondary launch method", when I save the configuration then go back to the configure page it has always reverted to "Launch slave agents via JNLP". (I want to use "Launch slave agents on Unix machines via SSH".)

I've tried several combinations of options but none have resulted in the 'Secondary launch method' being correctly saved.