In the unzipped folder is a folder called mmd_tools. Copy the mmd_tools folder into Blender's scripts/add-ons folder. After opening Blender, click File, User Preferences. In User Preferences, Add-ons, ...

How to edit an already existing bone pose morph: Select the armature object of your MMD model to make it the active object. Enter Pose mode. In the Morph Tools panel of mmd_tools, select the [Bone] ...