Alternative way of setting the debug mode
You can try setprop persist.service.adb.enable 1 in a terminal. This works on the HTC Dream; can't say I've needed to do it on other phones, and I can't say if it needs root or not (the exploit on the Dream runs that command as root, so it's completely possible you need root first.)

See this question here for other ideas: Broken screen while debug mode was disabled. How can I re-enable adb?