Its having trouble finding it on the network folder, USB Helper probably doesn't support Local Network Folders when creating Injects.
You can either use the tool SuperDan linked to in his post, or put the ISO on your PC with USB Helper and try it again.
To be honest, I rather download the Nintendont Channel (Not the Forwarder) for WiiU from USB Helper and install it with WUP Installer.
That way you don't have to boot vWii to use Nintendont, you can even use the Gamepad as the controller and the game screen, All of your gamecube ISO's are right there listed in Nintendont, you can configure settings and cheats before you start your game, and the games will not take up space on your Nand since they will be loaded from your SD Card (SD:/games/name_of_game/game.iso).
So its like having 1 single app on the Wii U main menu that has all of your gamecube games. And its very stable. I play a bunch of old resident evil gamecube games with it, and its been great, i even created some cheats for them.
The gamecube injects uses Nintendont too, the inject just bypasses the Nintendont GUI and takes you straight to the game. Its up to you.
You can find the Nintendont WiiU Channel app on USB Helper, just make sure you don't choose the Forwarder version. Then install it with WUP Installer. Also, make sure in USB Helper you "Configure your SD Card for Injects". Im not sure if that is necessary or not, but thats what I did, and it worked fine.