The Developer Menu is straight-forward. Go to the Safari Preferences / Advanced and enable the menu. The Debug Menu must be activated in the com.apple.Safari.plist. Either by issuing the following command:
defaults write com.apple.Safari IncludeInternalDebugMenu 1
Or by opening the plist with the Property List Editor Application and adding a new ‘IncludeInternalDebugMenu’ node, with a boolean value set to 1. Same thing, different means.
Don’t forget to restart Safari to see the changes.