As I said do not follow any online guide they are most like to be out of date a defunct and the risk is very very high for mess ups
http://modmii.zzl.org/ is all you need for the modding part (again do not use online guides [main reason is out of date info resulting in some games not working and manual ios/cios updates that is dangerous if you do not know what you are doing])
I will first ask what you actually have at the moment in hardware you can use
An SD card is required so do you have any SD cards kicking around (Micro/mini will do as long as you have a SD adapter for em)
USB devices the preferable type is a HDD but for now you can use a USB stick
To get files to the SD card can be done via plenty of ways (micro SD's for example can have files coppied to them if you pop em into a mobile phone or tablet, anything that can take a card and has USB to a PC setting the phone or whatever to use USB to copy files to from the micro SD via USB) [A digital Camera is another example]
This is only a temp measure as once the thing is modded and you are using a USB HDD you don't need the SD card afterwards
P.S. this game does require Motion Plus controllers or Motion plus expansion packs
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