This is how to load motion data, not download it.
The reason I made an ible of something so simple is because MMD beginners have no idea what their doing, and this is a frequently asked question. So, without further-a-do...
Step 1: Load Your Model.
Step 2: Select It
Step 3: Select "File"
Then, go down to "Load Motion Data"
Step 4: Select the Motion You Wish to Use
Step 5: Voila!
Now make a video of models doing retarded things like everyone else.
I listened to Faster Stronger Gaben while making this. I'm perfectly sane.