Re-uploading because other download gives error 403.R4 Injektor allows you to edit _DS_MENU.DAT without using a hex editor or tile molester. You don't even need to decrypt the firmware.
I don't have a wii u anymore to test it myself, but if homebrews are not visible on the wii u menu I think you can press L + R + minus to open the plugin menu, there should be an option called "homebrews on wii u menu" or something similar