Z axis not lowering to build height..

Hello
I have a diy 3d printer, same as a 3d systems cube 1. with ramps 1.4,. I want the z axis, plugged into z -max on the ramps, and I need it to lower when building height on a 3d object.. what setting in the configuration h file.. do I need to change.. Also I have limit switches, all in min, for each axis..
So I need the Z axis to home in min, on the printer, but be plugged into Z max...

Any help with this would be great.