I don't have any direct experience with your NAD M15 but according to the specs on the NAD site it has four optical digital inputs.
If your Macbook was made within the last decade it almost certainly has a mini-Toslink optical output available through the headphone jack. The simplest possible connection would be to get a Toslink cable and mini-Toslink adapter for the Macbook end. Plug the other end into the NAD and I would think you'd be set.
Can't help with whether you'd need to select and configure that input on the NAD, though. The DAC in the NAD is almost certainly much better than the one in the Macbook. Be sure to check the manual on how the optical inputs work.
USB out of the Macbook and routed through a USB DAC is also an option. There are a number of well-thought-of inexpensive DAC's like the Musical Fidelity V-DAC and the Nuforce that would make the connection. I use an Aune mini-DAC in my office with an iMac USB connection and it's very good.
If your Macbook was made within the last decade it almost certainly has a mini-Toslink optical output available through the headphone jack. The simplest possible connection would be to get a Toslink cable and mini-Toslink adapter for the Macbook end. Plug the other end into the NAD and I would think you'd be set.
Can't help with whether you'd need to select and configure that input on the NAD, though. The DAC in the NAD is almost certainly much better than the one in the Macbook. Be sure to check the manual on how the optical inputs work.
USB out of the Macbook and routed through a USB DAC is also an option. There are a number of well-thought-of inexpensive DAC's like the Musical Fidelity V-DAC and the Nuforce that would make the connection. I use an Aune mini-DAC in my office with an iMac USB connection and it's very good.