Isn't the NavMeshGraph expecting the "walkable" area? If so, how is this working for you? I've defined colliders for the wall sprites, which would imply that these are the areas you can't pass through. If I'm not mistaken, this won't generate an area that the pathfinding objects can walk on.
Am I missing something here? If anyone else would like to comment on how to build a navigation mesh out of tk2d tile map data, I would greatly appreciate it.