The MAC is not a 'layer' per se, but part of the link layer. In ethernet (assuming that is what you are referring to), the classical concept of physical layer and data link layer are more or less combined. One passes information to the link layer by writing data and control words to the appropriate registers in the hardware. This is the domain of a device driver. You can do this from a userspace application, iff you know how to program the hardware you have available, and if you have root privileges.