Currently, the plugin does not provide this functionality but will be implemented in the future version of the plugin.

Is there any progress on this feature?
I’m having a very similar problem and this feature would help a lot.
In the attachement pls. find an excerpt of my search form. What I want to do is: putting the Taxonomy ‘Herkunft’ in a drop down list and by selecting one of the items there getting all posts being taxonomied like this – in fact, it works more like a filter rather than a search.
Another possibilty might be to get the chance to search for a taxonomy name (like USA) and find all posts with this taxonomy. For now, there is only the possibility to include or exclude taxonomy names.
By the way, what you can see is my German translation of the plugin – interested in the .po and .mo files?