The sample is just an example of the "default" application, the "foo" entity is not complete.
If you use the generator, here are the steps to make the "foo" entity working: http://jhipster.github.io/creating_an_entity.html
There are basically the same steps to do on the sample application: you need to set up the database tables (in Liquibase, they are just commented), and import the JS files in your main app.
But you are right, we should generate a complete entity in the sample application (or no entity at all!), but not provide this half complete result.