Use a tool like Wii Mod Lite or YAWMM to browse to the file.
Its primary purpose is booting burned DVD-R discs (on older Wii models with compatible disc drives). Wii Neogamma R9 Wad
To use NeoGamma or its modern successors, your Wii must have cIOS (Custom IOS) installed. This acts as the "driver" that allows these apps to access the disc drive or USB ports in ways Nintendo didn't originally intend. Use a tool like Wii Mod Lite or YAWMM to browse to the file