Unfortunately, the Ergonomic keyboard config utility doesn't allow you to change the settings for the centre zoom button. In order to do so, you will have to edit the C:\Program Files\Microsoft Intellitype Pro\commands.xml file manually. (For computers running the more recent Microsoft Mouse and Keyboard Center the path is C:\Program Files\Microsoft Device Center\commands.xml)
N.B. the contents of this file change depending on which version you have so make sure you have the same version as me before you try. The version number can be found at the top of the Commands.xml file. My version is:- 8.0.225.0If this explanation gets too basic, it's because I don't know how much you know about Windows or config files.