There you find a button that says-'buy steem or steem poweer' then a pop-up will appear, then you have to choose what to buy, Steem Power or Steem. By default this is set to Steem, so you need to click Steem Power first. Then follow the steps. this is the, which guide you.
https://steemit.com/steemit/@oaldamster/how-to-buy-steem-power-the-quick-and-easy-way