Extract the zip folder. You should see a file ending in .vmd (motion data) and possibly .vpd (pose data).
The K-pop MMD community thrives on mutual respect. When you download a motion, you are typically entering a "silent contract" with the creator. Failure to follow these rules often leads to "DL Down" situations, where the creator deletes the file because of misuse. Kpop Mmd Motion Dl