Drag and drop a material into the Materials list to apply the material to a highlighted 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.
A UV material is being applied to the Petal layer, here.
You can use these options to choose the projection mapping of the texture used.
Offset and Length settings allow you to control the global scale and offsets of the material, while the Tile and Repetition values set how many times the image will tile.
In this image, the Projection mode is changed to Spherical, with 10% Length U and Length V set.