Firstly where is the Property Link located? It can be created by creating a GUI element then right click the GUI element on the tree, go to create then click property link.
The Property link can be used to create simple interactions between GUI elements and other objects.
In the example below the property link has been created on a slider and then linked to the Opacity of the Material (Please note that for the opacity to work the Blending on the material properties can not be set to None)
As the slider is dragged the material (and the objects it's applied to) with become more or less opaque.
In the example below is a button linked to the enabled property of a Cube (note that the button must have it's Toggle property enabled), when this button is clicked the Cube will be enabled/disabled.