I don't own the Steam version, but is there a separate application for the settings? Look in your steam/steamapps folder for the Saria folder and then look for the file 'Settings' or 'Settings.exe' (it could be shown as either one on your system). Settings.exe is where you can type in the code to unlock the full game, at least, in the non-steam version. I imagine it isn't very different for the steam version, though. Good luck!
EDIT: In response to the above edit, you should probably leave your question be, so that others who have similar issues can get help too. For example, instead of removing the content to your post, you should post what the solution was. I myself am curious if it's how I described or if there is an easier way in the steam version....