Emberjs how to add and use a property controller to change something in the view

I read this which you can see has a very simple example of a template action changing a boolean property in a controller.

When I try to do this.set i receive the error this.set is not a function.

So how can I follow this example? What's missing? What it 'this' meant to be?

Note that this property does NOT belong in the model, as different controllers may use this property for different things. For example see this and the comment from Mike Grassoit explaining that.