As an alternative for those with technical limitations, Mare Humboldtianum was included on the November Observing List as an additional option. I figured this meant it would be easy to find - but I was wrong! Mare Humboldtianum is so far on the outer edge of the moon it's barely visible as a little dark dent in the disk's perimeter.
The reason it's so hard to see is due to lunar libration - or small oscillations that tilt the moon away from its average position. Although the Moon is tidally locked to the Earth, it still wiggles a bit - sometimes due to our perspective and sometimes due to its tilted axis and elliptical orbit.