How to enable indexing in Xcode?
Defaults are a name-value store per domain. The setting's domain here is com.apple.dt.XCode. The setting's name is IDEIndexDisable. You set this to 1. To undo this, you need to remove the setting, not add another one with a different name.
Based on the command you entered the first time, use this:
defaults delete com.apple.dt.Xcode IDEIndexDisableWhile you're at it, you should delete the key you added as well:
defaults delete com.apple.dt.Xcode IDEIndexEnable