You need to transfer "Steem from Savings" to "Liquid Steem" first. You can do this by going to Wallet then click "Withdraw" from Steem from savings.

Bear in mind that there is a 3 day waiting period before it becomes liquid steem (or simply steem). After 3 days, you can now freely move your liquid Steem to Steem Power through the process called "Powering Up".

Just head over again to your wallet and click "Power Up".