The model to be calculated is the flow of the air around a circle, which could be the section of a cylinder.
Files->Open...
and select the cfd_2d_example.gid model.
...
Click on the book icon in the left toolbar. 
This will generate the geometry and the tree assignations. You can navigate through the tree and see the assigned values. Save the model, mesh it and run it. (It may take some minutes).