Go to the folder that contains Lightners Live.exe
Create a file called "enabledevmode" (no file extension, just that)
NOTE: This font doesn't have lowercase letters, but "enabledevmode" is in all lowercase.
Now Developer Mode should be enabled. If it doesn't work, try restarting your game. If it still doesn't work, make sure the file is in the same folder as Lightners Live.exe, is spelled exactly right, with no file extension, and has no uppercase letters.