Well, personally, if I want something to be transparent in the modelling tool I use Milkshape3d, I just switch the faces around so they appear black in Milkshape's Render, then, in Blitz they appear to be invisible..
Also, on my earlier post about the ladder, I was having difficulties with climbing up the ladder, so in Milkshape3d I loaded up the model, and put a box around it and scaled it so it was a tight fit but slightly larger than the ladder, then I created a new material and assigned it to the box, and gave it transparency, so, it works :-D
Just my 2 cents worth, someone probably has a way better answer than that way.
Also, on my earlier post about the ladder, I was having difficulties with climbing up the ladder, so in Milkshape3d I loaded up the model, and put a box around it and scaled it so it was a tight fit but slightly larger than the ladder, then I created a new material and assigned it to the box, and gave it transparency, so, it works :-D
Just my 2 cents worth, someone probably has a way better answer than that way.