Well, thats easy. Select the object you want transparent and choose a color you havent used on the model. Then rightclick that color and click edit material. In thewindow that pops up you see the transparency slider. You can also adjust color and stuff there. Just set the transparency you like and close. To get the transparency to work as it should you need to always keep the object at the bottom of the hierarchy. Just drag it down. Save and close Ac3d Reopen the model and there at the end of the materials is your new transparent material. I dont know why it is like that, it just is