Starting with NetLogo version 5.1.0, the use of the Java applets is unfortunately deprecated, as explained here; we plan to move in the future to the NetLogo Web feature, to restart publishing these essays in an interactive way.
So nothing working on line, but it is possible to download the introduction to the Fishes-fishers-consumers - How to model their interplay simulation work and its .nlogo file (to download, it is better to right click).