Enker is only available on the latest version, he is a secret stage.
To unlock him, you have to:
1. Find 4 secret letters scattered in the levels of Cut, Ice, Fire and Elec-man (The 4 robot masters from MM1 that were in Wily's Revenge), most require a specific weapon/item/chip to access.
2. Once you have the 4 secrets, head to Wily stage 1 and find a teleporter, it will only open if you have the 4 secrets, once you step on it, you will be teleported to Enker's stage.
3. Beat Enker's stage.