Deselecting the Color option when creating a pie chart causes percentages not to show. While there will be space for the percentages in the legend, they will not appear.
A simple workaround is to leave the Color checkbox checked while creating the chart. After clicking OK, the percentages will appear in the legend. Double-click the chart to edit the chart options. Now, click the Color checkbox so that it is unselected. Click OK and the percentages will appear in the chart.
If the chart has already been created, double-click on the chart to edit it. Click the Color checkbox so that it is selected. Click OK. The percentages appears. Double-click on the chart a second time to bring up the Chart Options dialog box. Click the Color checkbox to uncheck it, and then Click OK. The percentages now appear in the chart.