Drag and drop a material into the Materials link field to apply the material to a layer.
You can select multiple materials and drag them in or, alternatively, use the eyedropper tool to swatch them directly from the Cinema 4D Material Manager.
Clicking and highlighting the material will then give you further options below.
In this image, the material has been dragged and dropped into the Materials list and is only affecting the selected Leaf layer.
By clicking on the drop-down arrow, you will see a list: Stem/Midvein, Vein and Leaf.
This makes it possible to assign the material to an individual part of the leaf.
Simply highlight the material, then click on the correct part in the drop-down box.