Well..... I figured it out.
Here is what I had to do.
In the editor window next to the tracks is a collapsed list of the VST Instruments that I have loaded. This is where the Ambience has to be applied, under the "Inserts" tab of the inspector for the VST INstrument NOT the midi track that has the actual performance data. That is what confused me. There is a separate collection of effects for the MIDI track and Ambience is not found here!
There are two or three ways to run your mixes through a reverb unit.
1) Use an Insert on the master buss
2) Create an FX Channel with Ambience as an insert, then use the "Send" on each instrument channel strip to send some portion of the sound to the FX Channel, or
3) Create several "Group" channels and then submix things with separate Ambience inserts per group, one for strings, brass, percussion etc.
The first two only require one instance of Ambience for the entire mix. The other means you're making adjustments for one reverb instance per section. You can go all the way to the track level with inserts per instrument, but that strikes me as a waste of resources and highly unmanageable when you need to tweak parameters, and are forced to do the same for every instance.
The bottom line: consult your Nuendo manual for more advice on how to set up effects. There's lots of good information on how to handle the basics.