Have you installed the "Zadig" drivers ? (It's required that you install that driver for your PC to detect the GameCube adapter.)

If you have Dolphin version 4.0-8601 and newer you can further save yourself some pain by just simply selecting the GameCube adapter for WIIU option under the GameCube controllers option in whichever port after you finished installing the drivers ...

The below are the outlined instructions at the official wiki ...

https://wiki.dolphin-emu.org/index.php?title=How_to_use_the_Official_GameCube_Controller_Adapter_for_Wii_U_in_Dolphin#Dolphin_Setup

I'd just get a Mayflash adapter and go on with my life. I use one of their adapters with my Pro controller, and it makes it recognised just like an Xbox gamepad. Saved me a hell of a lot pf frustration.

Looks like their GC adapter isnt even very expensive. Just $15 on Amazon.

Damn! Double post.

Still doesn't work. I tried to uninstall and reinstall everything, ty all of the USB ports of my computer, use the automatic fuctionon the Dolphin emulator... nothing. The computer detects the USB adapter, but no matter what I do, the controller isn't (and the controller isn't broken, I tried it on my Wii).