You need to create a copy of your wii memory (it's a lot easier and faster than doing a clean new dump from modmii).
You should look for "Simple FS dumper", but if you don't have it or don't want to use it, you can use modmii to generate a new clean NAND.
running a clean nand for the first time is like booting the Wii for the first time (asking the region, the date, synchronize wiimote, etc.), it takes ~10min to boot! (black screen for 10mins)
you need to place your NAND dump here :
USB:/nands/<myNAND>/ (rename <myNAND> like you want)
example :
USB:/nands/43u/
USB must be the first partition of the drive.
To boot into neek mode, you need Nswitch homebrew or channel, or use a USBLoader (like wiiflow or USBLoaderGX)
or you can boot into it at console power ON if you have Bootmii installed as Boot2 (but it's only possible on very old Wii)
Are you already using any USBLoaders?
making a NAND dump and booting neek is a lot easier from the loader.
What you should do is :
1. generate the "copy to USB" from modmii and place it to USB.
You don't need the "copy to SD" if you generated Uneek+di (U is for USB, S for SD) and you don't use bootmii to launch it.
2. go to usbloader (for example usbloadergx), and select a EmuNAND path in the settings to be on USB (anywhere is fine, usb:/my_super_neek_folder/)
3. make a NAND dump using NAND dump option inside the loader
4. Set the emuNAND launching mode to "neek"
5. run your game channels located on neek from the loader
note: if you are on WiiU vWii, modmii will not work.