In the shader room, set the Transparency channel to MAP and then load it the same way you would a texture map.  You can also do things like mix it with a color channel or multiply it with a color channel to make only certain colors transparent using the map.  It's too bad that Daz hasn't gotten around to releasing a real user manual for Carrara.  It would help out a LOT of new users.  Most of us learned on our own, the hard way.  Don't give up.  You will figure it out eventually.

Quote -  OK, but how do I load a transparency map?  In Carrara, in the shader thing, the transparency option has no place to load a map. If I select "texture map" it uses the texture map as a transparency. I can't figure out how to have separate texture and transparency maps.