No worries! So, I used the actual cave/door graphic (which is solid black and does not stand out) from the game, but it is hard to see. After you kill the harpies go into the cave directly in front of you. On the far right of the map is a black door shape against a wall. There is also a dark blue water puddle in front of it. Once you enter that cave you will go into a cut-scene, and after you just walk down and out of Mt. Fubi.
Please let me know if you are still having trouble, and I will be fixing the doorways next patch.