map transparency color

hi, i want to set a color as transparent. the help file says that there is a Map Transparency button in the color palette but i just can't find it. in my color palette, there are only three sliders for R, G and B, another slider for colors, and the two squares showing foreground and background color only. where is the map transparency button?
thanks much in advance.

I am having the SAME problem. No one seems to be able to help me. Have you found out??

I found what I was doing wrong...
I went here:
http://www.heathrowe.com/tuts/knockout.asp?knockout=save4web
And realized I was not choosing a preset. Once I did that, the color table opened. However, it didn't remove enough of the color for me. Artifacts were left.