PMD/PMX are the character/accessory format, and VMD is the motion format. Thes tools show up on the Blender tool shelf (they get their own little tab) and allow you to not only import and export the characters, but also the motions. It's a quick way to get MMD dance routines into 3DXchange by going through Blender. Just export as an FBX from Blender. I just tried it with the 2.76, and it works great.
https://github.com/sugiany/blender_mmd_tools

Don't forget to find a T-pose for MikuMikuDance.

Here's a jumping-off point for MMD characters on DeviantArt
http://pmd-editors.deviantart.com/gallery/30850289/Downloads

And here is jumping-off point for dances at same.
http://mmd-nay-pmd.deviantart.com/art/MMD-Motion-Data-Index-A-G-Updated-14-04-15-305111287