There are a few different configurations that could cause the hum, but we need to know more about the outlets, power cables and general setup.
I'm starting to think it may be insufficient grounding on the laptop end. If the amp is well grounded, the path of least resistance could be going through the USB cable, DAC and then to the amp. When you touch the macbook, the path of least resistance changes to your body, thus the noise goes away. The trouble here is the ground wire coming from the wall doesn't go directly to your laptop. It's usually somewhat isolated inside the laptop powersupply so it's not a direct ground. The ground on your amplifier end may be direct to the wall.
Take a look at your laptop powersupply and see if it's using a 3-prong grounded plug. If so, make sure you're getting a good tight connection in the wall or powerstrip. Make sure your powerstrip has a good tight connection into the wall. If possible, use the same powerstrip for your entire audio system to avoid any differences in ground quality from using different wall outlets.
If your laptop has a 3-prong grounded plug and the above suggestions don't work, then bypassing the ground with an adapter on your amp will most likely solve the issue. It's perfectly OK to try this as a diagnostic tool to find the problem. It's up to you if you want to run it this way long-term. Personally, I would without hesitation. But then my house has no grounds at all so it's a moot point.