EDDM doesn't have the customize option, because it's already been customized by us.
Check the fsdreamteam\Couatl\GSX\airports folder, there's a list of all airports that have been already customized, so the option 6 won't work there.
If you remove the relevant *.pye file, it will work but, keep in mind you'll lose our customization (so it's really not something you want to do with FSDT airports), and in case of a Live Update, the file will be restored.