Snapshots are stored in a hidden directory called .snapshot at the top level of the volume. For example, the snapshots of a volume called MyVolume are in the MyVolume/.snapshot directory. The directory is intentionally hidden from tools like ls to prevent recursion. You have to know .snapshot is there to find it! However, once you cd to .snapshot, you can list the contents normally.
To learn more about volumes or any of the other features that MapR has to offer, visit MapR Documentation.