first, fix your missing dol file (place usb loader GX here : \apps\usbloader_gx\boot.dol )
then, know that USB Loader GX is using a slightly different NAND dump format.
Dump done with SimpleFS dumper, or extracted Bootmii dump are not working correctly. The reason is that the FAT32 format doesn't accept some characters (these one: "/\*?|: )
Some games are using these characters on the Wii's NAND, for example for savegame's filename.
In order to store this files on a FAT32 drive, these incompatible characters have to be renamed and automatically converted when the Wii is reading the EmulatedNAND.
The conversion process is different between homebrews/cIOS.
Dump working with Mighty Channel will not work 100% with USB Loader GX.
You need to create a new dump of your Wii by using the integrated "NAND Dump" feature in USB Loader GX settings.
The default path for extracted NAND path is : SD:\nand\, but you can of course change this path in the loader's setting.
Your main problem with your current dump is that the Nand is on SD:\ and not in SD:\nand\
So either:
1. Redump your Wii nand by using the loader's dump feature,
or
2. Try pointing the "Emu Nand Channel" path in the "User path menu" to SD:\
Be aware that not all wiiware or VC games will work.
Edit:
Also, USB Loader GX can list and launch channels directly from the NAND and the emulated NAND.
You don't need to create ISO Forwarders anymore, so don't follow your tutorial.
1. install GX
2. place the wad on your SD card (anywhere, but usually in \wad\ folder)
3. launch GX and create a dump of your nand
4. Go to GX settings > Feature > Install wad
5. on the main screen on GX, click on the icon with "wii box/channel" and enable the "Channel on emuNand" checkbox.
You need the latest beta version of USB Loader GX (1156 mod or newer) + Latest cIOS (v8 or newer).