Short explanation. In material group settings enable May be veneered.

Longer explanation. Make sure May be veneered is enabled in material group settings. Then, in the element editing window, additional four rectangles will appear on the diagram. By clicking on them with either the left or the right mouse button you can set the veneer type.

Curious fact. You can also navigate the element diagram by keyboard. Use arrow keys to move and the space bar to modify the values. Additionally, when an edge is selected, you can press the MENU button (between ALT and CTRL, on the right hand side of the keyboard).