To buy Power Buff, open the "D" menu in the game and go to the second page. This feature only works in town.
Select the Power Buff duration that suits you and click the "Buy" button.
Warning: the game does not ask for confirmation after you click Buy. Be careful!
After the purchase, you will be taken to the main menu of the game client. When you return to the game, you will see the Power Buff icon; hover over it to see the timer.