uh, you can't? Because he's not(yet) a playable character, he only appears in Konquest. Where did you get the idea he's unlockable in Practice mode?

Johnny Cage is only playable through a Pro Action Replay, Ill bet he cannot be unlocked by any normal non cheating device means.

Someone found a glitch in Practice Mode where if you first selected Scorpion's alt, then picked anyone on the top row and held start and up, it would actually pick a totally different character than the one you were on, and even if you didn't have their alt you could use it through this method. This was proven true, as I have done it myself. It has no real significance though.
Rumors started that through means of performing this glitch over and over and over, it would actually give you Johnny Cage. Even if this was true, it's pretty useless in Practice Mode, but I do believe it's since been proven false.

that's not a glitch, that's just a random select... it's been in every MK, including DA... and the Johnny Cage thing is BS, the only way to play him is with an AR

No, the random select isn't the part labeled as the glitch. Know what you're talking about before striking things down.