Here's a post that explains the 'why' a bit more on why we're being paid in only Steem and not Steem Dollars right now.

The other comments are correct though. You can exchange your Steem for Steemdollars using the internal market here. To do this, simply click into your wallet and then the drop down menu next to Steem. Select the option to Buy or Sell.

On this next screen, you'll see the Buy's on the left and the Sells on the right. While you can easily click the small links of 'available' and 'highest bid' (which will populate the Fields with you total Steem and the highest current buy price) under the 3 sell lines of Price, Amount and Total, you can set these to whatever your want (i.e. not sell all Steem in your wallet and/or change the price to something higher than the current bid price, even though this may take time, or not sell, depending on the market.)