Yes, Mismatch Endonuclease I requires MgCl₂.  MgCl₂ is present in NEBuffer r2.1, so if Mismatch Endonuclease I is used with the supplied NEBuffer r2.1, no additional magnesium needs to be added to the reaction. If using Mismatch Endonuclease I in a buffer other than NEBuffer r2.1, supplementation with magnesium may be required for optimal enzyme function.