This is because you have just added the add code on your site or there is no publisher who is interested in showing the add on the keyword you are targetting. However, it is suggested to re check the ad codes you have pasted on your page. Also, wait for a while or browse other pages to check if the adds are displayed properly.
Final words: If you have pasted the code properly, you don't need to worry about the adds or blank space. Allow some time.