I couldn't find a cheat code to unlock Eliot, aside from a code I found to unlock everything. But when you say achievement, do you mean the Robots Doin It For Themselves achievement? There is no achievement for unlocking Eliot. I don't see how that cheat code would affect the achievement I mentioned. Unless, of course, inputting a cheat code disables all achievements.
I believe in the rock band series if you turn on No Fail mode it turns off all achievements. So I'd be wary of using any in Dance Central, since it's also made by Harmonix.