Go to the mmd_tools download link below then click "Download ZIP"
Extract ZIP File.
Now you have a new folder that is named "blender_mmd_tools-master". Go inside that folder once
Cut and paste the "mmd_tools" into your addons folder.
(EX: Program Files\Blender Foundation\Blender\2.76\scripts\addons)
Open up blender and go to "File" then "User Preferences"
Once the Preferences box pops ups search "MMD" and to your right "object: mmd_tools" should be seen. the next step is important
Check the box next to the little running man and at the bottom click save user preferences
(If you don't you're not going to be able to import .pmd or .pmx files)
Go to "File" then "Import" and import your MMD Files.