So I installed USB loader gx and I downloaded an iso (ssbb - 7.85 GB). It's a .rar file. Do I have to unzip this or put it as a .rar file on my sd card? And where do I have to put this? Thanks in advance. Edit: Thanks for moving this, Reider.
You will have to unzip it and put it in a HDD that's compatible with the Wii and format the drive to WBFS format using WBFS or Wii Backup Manager. I think you have to change the ISO to WBFS format also and I think that's possible with either managers.. And also the SSBB backup is bugged so when you play single player you see almost every video is the same.
You can use a 16GB pendrive (basic Sandisk is ok for me) if you are only interested in a few games. It is faster and cheaper than a HDD. I recommend you to use WBFS Manager. You have to open wbfsmanager.exe (in windows). Be sure you format with this program the unit you want to use for the Wii. When formatted, add the iso (that previously, as I-Fatman said, you have had to unzip from the .rar). Close the program, take out the pendrive or HDD and plug it to the Wii