If you don't mind me asking, why do you want to activate it? The feature is used to force the card to run at its max speed in programs that would normally not run it with turbo clock or not detect the OC, for example. Unless you're doing benchmarks and you notice the card not running at it's full potential, then you don't need it.
To answer your question, there are certain programs that have to be closed for you to active Kboost. I believe Precision X tells you what to close, but I'm not sure.