That's the reason you can't enable 120hz. Only a mini-dp to display-port will work or an active (ie, usb-powered or powered in general) mini-dp to dual-link DVI cable. The later is very expensive and i'm not 100% sure if it would even work. From what i've heard, they have tons of issues such as the screen turning off intermittently, flickering, and you have to unplug/replug the monitor frequently to fix it when it blanks out. Try to get a mini-dp to display-port. That should work.
Edit: even the powered ones don't all work. For example, this ACCELL adapter is powered and it says it doesn't work for 120hz. It honestly should work though, i don't know why it wouldn't.
Edited by SeanPoe - 12/20/11 at 9:41pm