First, open your AdBlock menu from the icon. This should be in the upper-right corner of your browser window (I'm using Chrome on OSX in the example). Select "Options" and you should see this menu towards the botton of your screen.
www.tumblr.com##DIV[class="sponsored_post"]Click the "edit" button, add the line of code, and then click the "paste" button. This will block all sponsored posts that appear on Tumblr.com.