How is dark energy consistent with conservation of mass and energy?

The total energy in the space does increase, precisely because of the reason you mention. Energy is not expected to be conserved, because the metric is not invariant under time translations.

What does hold is the first law of thermodynamics, $dU = -P dV + \cdots$. Since the pressure in this system is negative, this is one way of seeing the origin of the extra energy as the space grows.

"I presumed the source of this energy was not coming from the conversion of other types of energy to dark energy, so it must violate conservation."

This is where you go wrong. The positive dark energy is balanced by the negative energy in the gravitational field. As a volume of space expands more dark energy is created in the volume but this is balanced by a growing negative term in the energy equation that depends on the rate of expansion. That is why dark energy causes the expansion to accelerate.

The reference Carroll, Press, and Turner (1992) explains this in more detail, or see my own paper at http://vixra.org/abs/1305.0034