Okay, yes found it and it works. I think this should probably be mentioned in the first post or on the installation instructions here http://kerbalspaceprogram.com/mechjeb/. Thank you.
I have tried to install MechJeb by copying the contents of the .zip folder into the GameData folder, but when I start a new game the mechjeb button doesn't appear on the right hand side. Does anyone know what might cause this? Thank you.