In order to use D values with G41/G42, you need to turn the Tool Offset Registry on in Settings.

You will then get a Tool offsets tab in the Inventory browser (F2).

On the picture below, we have made a program illustrating how the D parameter affects the cutter compensation. In the example, we have put 

30 as the D geometry value.

For more information on the topic, please have a look at our online help page about radius compensation.