For a standard form/subform, the control reference syntax is fairly straightforward.
See e.g.
http://allenbrowne.com/casu-04.html
However referring to controls in navigation forms can be rather difficult if you alter the basic design in any significant way such as using nested or tabbed subforms. The reason is that they are not standard forms
That is one reason why many developers avoid the built in navigation form (similarly split forms)
I would suggest you do one of the following:
1. Save the selected combo value as a public variable (or tempvar)
If using a variable, create a public function to grab the value.
Then use the function in your filter criteria for the next stage
2. Design your own form/subforms that look like the navigation form you have now but behave like standard forms