Are you talking about the MM enamels or do you mean the MM Acryl line?
The Acryl line, while I'm not sure if water based, can be thinned with rubbing alcohol or distilled water. I use distilled water, myself. I use Windex with ammonia (ammonia being the key ingredient) for cleaning my airbrush.
The enamels are not water soluble, so they need either the MM proprietary thinner and cleaner, or you can use mineral spirits or lacquer thinner. Same chemicals for cleanup.
HTH,
Chris