Hi Boardwriter,
One possible reason for your problem might be that you play SS2008 in an older version. Jumbo Javelin needs the version 1.4.2 which is, as far as I know, the latest one available.
If your game version should happen to be an earlier one, you'll find the necessary updates on the Ship Simulator home page.
Best wishes
Volker
aka Eustache le Moff