You're going to have to experiment a little bit, but orde already pointed to the approach to take that will probably work.
I suspect that something along these lines is probably the right answer:
browser.div(id: "overview").span(class: "dijitTreeLabel").fire_event "onfocus"
You may have to follow that line up with another one that does the click...
browser.div(id: "overview").span(class: "dijitTreeLabel").fire_event :click
...again, it's hard to know definitively what's going to work, so you'll have to experiment.