You need to have RePast installed in order to run this model.
Download and extract the content of the geosim0.zip into your project directory (compmodels).
Within IntelliJ IDEA, go to File->Settings->Paths and click on the button to add a module. Select to import and existing module and specify geosim0/Geosim 0.iml.
Go to Run->Edit Configurations and create an application definition with the appropriate settings (the most important value is the main class, which define the entry point to the program; in this case this is ch.ethz.icr.geosim0.ModelGUI):