So, as you might be new to MMD, you may ask, how do you install stuff?
Well, today, I'll help.
First, let's say we're downloading a model from this site. You download it from mediafire. And then you get this file.
It's in a folder. You dont have to open the folder.
Just copy the folder into the "Model" file in your MMD. When you run MMD, open the folder and select the ____.pmd file. If your model opens correctly with all of the colors and glory, you've done it!
Same thing can go for .wav files, or .vmd files, etc etc. It's like this, except no folder. Just copy the file into it's respected folder.
So, that's about it. Happy downloading!