Originally Posted by BH_MAN

Hit Menu on the Phone ( Not settings in the Launcher ).

Menu > Settings > Applications > Development > USB debugging

After a Phone reboot, be sure to toggle off, then back on.

If you're running a Full stock Rom....
Pull down the Menu bar ( Notification bar )... and

Notification Bar > Phone Tab > USB settings > Samsung PC Studio
( Then follow the USB Debugging how-to ).