I was shocked at the lack of MMD tutorials on this site! Seeing this as an Internet sin, I have decided to make one myself.
How to install models in the latest version of MikuMikuDance.
Model used: Shamolka's Sonic Miku; Gordon Freeman by BloodyKylie
Step 1: Find Your Model
I recommend using DeviantART, since it's the best place to find MMD material.
Groups in DA I suggest: MMD Mall
Step 2: Download Your Model
Click the download included in the desc of the model. Sometimes, the files are password protected. To avoid this, use the password given in the desc by the author.
Note: You cannot download most models if you have AdBlock on.
Step 3: Open the File and Drag and Drop the File Inside
If your download was a .RAR, extract it with WinRAR or 7zip first, then drag the folder out. If there's no folder inside, just the textures and bases, then extract the .zip file. Delete the original file.
Step 4: Voila!
You now have a brand new model in MMD! Now make it do Gangnam Style with Gordon Freeman.