Below is a texture that I attached to a Rect object made in AC3d
called testtran.bmp:
I then created another another bmp for the transparency
called testtran_a.bmp
This file is not referenced in anyway to the ac file created in AC3d I just made it the same size and made sure the name is the same but with the _a added to the end of the file name. Both images are an 8 bit bmp(256 color)..
Black (0,0,0) is tranparent, white(255,255,255) shows the texture map.. shades of gray in between the two seems to affect the level of transparency
This is what it looks like in the object editor
another one...