The Myo Developer Kit (MDK) is a Myo armband that was shipped earlier to developers. These units shipped only with access to the Software Development Kit (SDK) as initial software. An MDK has the same hardware as a Myo armband so firmware updates are shared for both units.
As of November 21, 2014 Myo Developer Kits are no longer being sold. You can order a Myo armband here.