I never activated it before I put it into my green drink before, but I am now that I have read this book!
However, I will say that I usually added a squeezed fresh lemon wedge right into my drink, and the acid from that I believe activated it; also the greens themselves (wheat grass, barley grass, etc...) are slightly acidic.
I had read enough about sodium chlorite back then to know that it needed to be activated; I researched it myself, and whenever it is used as a disinfectant, for water purifying, etc... they add a weak acid to it to bring out the chlorine dioxide.