It's on by default (and can't be turned off) in all modes other than Power Mode (where the MMB is configurable via Preferences).
If it doesn't work for you then your mouse drivers are probably not treating the middle mouse button as a middle mouse button. See this FAQ: Logitech Middle, Back or Forward buttons don't work.[/quote]
I´ve turned that on, it´s not bad for using along with the one click mode. Only it´s prone to mess up the selection a bit, when klicking not properly (using that Logitech wheel-middle-click).
It´s not Dopus´ fault, though. 