That 10mm nut is only for adjusting handle free play. Free play is the play before the cable starts moving at all, which means it should be less than the 1st click when you pull the handle. Adjust the handle free play first, by loosening the nut all the way until the handle feels ultra loose, like only gravity's keeping it in place. Then tighten the nut until the cable get slightly tighter, then back off a bit. The play should be about a few millimeters.
I find it easier to remove the center console, and remove the ebrake handle cover so I can use an open wrench on the 10mm nut while the handle is all the way down.
To adjust the drum brake itself, you need to go to the rear rotor. Release the handbrake all the way. There's a hole on the rotor hat where you would insert a flathead and turn this little adjuster nut with notches on it. Turn just until you can't turn the rotor anymore, then back off 2-3 notches. Repeat with the other side.
You can look it up here, Brake system section, page 23-24http://
www.300zx-twinturbo.com/cgi-bin/manual.cgi