QTR-Quadtone RIP

I mean grey, the picture in photoshop has nice darker grey and lighter
grey, on the print, same grey all over.  Blacks are not really black
either.

OS: Mac OS X 10.4
Printer: Epson R1800
Ink: MIS 3MK (3 carbon inks)

How can I increase the contrast on the print using the curve? 

Secondly, and this may sound obvious to you guys, how do I tweak the
curve? I don't mean tweaking multiple curves like the UI allowed me to
do with the slider, but to actually change the shape of the curve itself.

Thanks in advance.
Will