Apologies for bringing this old thread back to life, but I had this warning and didn't see elsewhere how to solve it.
It's not an error, and doesn't cause any issues to my knowledge, I was just getting annoyed seeing the warning every time I compiled.
Here's what I did:
In iTweenLookFrom.cs, surround line 30 (public iTween.EaseType easeType = iTween.EaseType.linear;) with...
#pragma warning disable 0436
public iTween.EaseType easeType = iTween.EaseType.linear;
#pragma warning restore 0436