I'm having this problem where upon entering the cell, well the marker is already in the wrong place and i'm a total noob on how to move it. But even so when it starts the character just falls through the ground. Perhaps because it's in the middle of the ground? I don't know. I'm only on the nav meshing creation kit tutorial on youtube from bethesda. If an explanation on where to put the entrance markers comes up later please just say so and i'll find out later!