Click the + button to create a new piece of data. First, enter the name, then a number. The numbers don't have to add up to 100. When you've finished that, click Next. Now, click any color on the color picker, then click one of the chunks of the chart to set it to that color. When you're all done, click Done to hide the color picker and cursor.
Happy Pi Day! To celebrate, I made a pie-chart generator. CREDITS: • Color selector: http://scratch.mit.edu/projects/illusionist/504102 • Text: http://scratch.mit.edu/projects/11850094/