You also need to make an xml file for Riivolution to know which file to change. Just get yourself a Riivolution patch and look at the files

Riivolution works by loading Files on your SD card while running the game instead of loading them from the Disk. In order to work properly, you have to build a patch for the game you want to change. In order to make a new patch, you have to make an xml file where you specify which files you want to replace and where the new files are located.

The easiest method for you is to use the Riivolution template for MKW which you can download here.

Just put the xml inside the riivolution folder on your SD card and make a new folder called mkw on your SD to store your common.szs file.
This will make Riivolution load everything you have in SD:/mkw when using the patch.

Alternatively you could use your Mystuff file but you might have to change the file to work with Mario Kart Wii

It works for me. You should be able to download it with right click -> save as on this link

Thats literally the My Stuff xml way.