Applying Material Functions
Once you have created and edited your material function, you can add it into an existing material by dragging it directly from the Material Editor Palette, or from the Content Browser into a Material Editor. It will contain the various inputs and outputs defined by the input and output nodes within the function.
Alternatively you can hold down the F key and click in the material editor with the function selected in the Content Browser. If no function is selected in the Content Browser, this creates a blank MaterialFunctionCall node.
You can then assign an existing material function via the properties window and the Material Function property. Simply select the desired function in the Content Browser and click the arrow button next to the assignment slot.