To my understanding, you need to purchase bitcoin, and send it to the bitcoin address listed in the "Buy Steem Power" page. Admittedly I've never purchased bitcoin nor steem power myself, but I plan to purchase some bitcoin within the next few days in order to purchase some steem power for myself. It seems like a very worthwhile investment!

Edit: If anyone has a more concrete answer to OP's question I would also love to be clued in! :)