I don't know how 2D to 3D works, but I DO know it is impossible to change direction.
...I think.
2D mode is hardcoded to move along the X (horizontal) axis. You'll just have to build your levels around it.
Ok, as for your question on how to use 2D mode, you don't tag it directly to the sector. Tag a linedef type 300 to the sector, and then give the linedef adjacent to that one type 432/433. It should work then.
In 2-d mode you can trave l only travel on the x-axis If want it to travel on the y-axis you have to rotate your entire level so the platforms are on the x-axis