Hey and welcome to the forum! Yeah so only regular SD cards are compatible with the Mii Installer application. Most SD cards are SDHC, so most SD cards aren't going to work with that application. I see that you're using the right application now, but I thought I'd just explain why that is. Can you try changing both the client ID and avatar ID? And can you tell us exactly how you're frying to install your Mii?
I would also step in and say that the client ID cannot be the same as another mii! Just make sure to change that each timeAfter all this time It worked,I just had to change the symbol at the end of my Mii's name, for some reason I tried it with a pixel Heart and it didn't work. But when I replaced the heart with a full star at the end of the name, it worked.
For some reason I can only put some symbols.