I tried launching the app, I selected the cios and whatever. When I selected to mount USB, it just returns to the home menu. Anyone else knows how to fix this? If so, that would be very helpful.

Well, my hard-drive was already formatted to FAT32 in the first place. Let me try ModMii before you tell me anything else.Could be you have a suboptimal cios installed. Please share a Syscheck or just use ModMii.
Could also be that the hard drive isn't compatible or it's formatted improperly (fat32 required)
You could also just use an SD card if you have one available

I did. And it says that its not compatible for 4.3E even though its the same region and version.Describe the error you see when they don't install?
You are trying to install csm files right? Not the mym files? And you are sure you created the csm based on the correct system menu version?

I did install the right app. And I did check that the version of the theme was 4.3E.Have you done this successfully before? You need to use the correct .app file from your system menu and convert the mym to csm. It's quite easy to make a mistake.
Also, some mym files have been constructed to only work on certain system menu versions. For example, many older themes worked on 3.2, but required some underlying updates to work on 4.0 or higher. So if you are trying to convert an old mym to a 4.x firmware it may just be that the mym doesn't support it in its current state
Ok guys, I'm back at this post but I forgot do do a syscheck last time so here it is:I did install the right app. And I did check that the version of the theme was 4.3E.