Well, Duesentrieb is the guy who could explain things far better, but he is on holiday now.
I think, the Gizmo can switch one function only with one switch. So you have to have an insert cable (y-cable), which goes stereo to your jack, where you plug in your original Diezel footswitch (it is stereo!!!). The other two plugs of the y-cable you have to plug into two switches of the gizmo.
When you want to change modes of channel 1, you have to do the mod and after that you will have anothe stereo jack, which is connected to the manual switch on the front. You do just the same - an insert cable, which is stereo on one side and two monos on the other.
Nobels MS4 is much cheaper, when you fin one on ebay, and does the same.
Another possibility is (what I would recomend) - get two Rocktron Multivalves or Replifexes or G-Majors (or mix it). Each of them has a switch, with wich you could switch two functions. Just plug in one stereo cable to the footswitch jack of your Einstein and to the switch jack of the e.g. Replifex or G-Major and your done. It is much easier. But you need two devices, when you want to switch channels, master and the three mods.
But you have additional effects - and two used Multivalves (Raplifexes, G-Majors) are not that more expensive compared to that Gizmo.
My Einstein is fully midi-switchable that way.
Good luck!
eddie