if you have a gamecube disc, it's easier to just launch USBLoaderGX and put the game in the wii. It will ask if you want to install it. Select Yes, and it will dump and remane it correctly.
You will then see which path is used.
SD or USB:/games/whatever you want/game.iso
only the red part can be changed.
/games/Mario Sunshine/game.iso
For loader's loading speed, it's best to also put the GameID inside the red part.
/games/Zelda wind Waker [GZLP01]/game.iso
/games/Tales of Symphonia [GQSEAF]/game.iso
/games/Tales of Symphonia [GQSEAF]/disc2.iso
Like you see, the filename must be either :
game.iso
or
disc2.iso (if you have games with two discs, put both game.iso and disc2.iso in the same folder).