If the second indicator light on Webjimbo's status page is red, Webjimbo hasn't been able to configure your router.
Have you used the AirPort Admin Utility to turn on your AirPort's NAT-PMP? Here are instructions for Tiger (OS X 10.4) and Leopard (OS X 10.5) on how to do this.
If NAT-PMP is on, perhaps your router has allocated your Webjimbo port to another program. Try changing Webjimbo's port.
This probably means that you need to turn on UpNP for your router. You do this in your router's admin interface - this is usually a web interface that you connect to with your browser. Exact instructions will differ from one router to another and are not given here.
Perhaps your router has allocated your Webjimbo port to another program. Try changing Webjimbo's port.