fatslob-:O on 30 March 2016
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
