To mount an ISO read this http://madpenguin.org/cms/?m=show&id=451
To install directly from the CD/ISO, once mounted navigate to wherever the rpm's are on the CD/ISO (sorry I can't remember exactly where they are on Mandrake, but they'll probably be in a folder called RPM) select the rpm you want, I think you can just double click it, if not, right click and there should be something like software installer or something like in the menu.
That should be all you need to do. But why would you want to install direcly from the CD? Why not use rpmdrake from the GUI?