To unlock Max-D, you must score a Silver medal or higher on Desert 4 - Survival. This won't be hard, as the map is relatively low on tricky barrel placements. I finished in 0:20:789 and got a Gold medal on my first try. Another player hit 0:24:3 and got gold as well.
To unlock Desert 4 - Survival, you'll need to progress through Time Attack, completing Desert 3 - Time Attack.