To add a symbol to a data series using the Chart Properties designer, complete the following steps:

1. Right-click on the C1Chart control and select Chart Properties from C1Chart's context menu.
2. Select the Data element in the treeview pane of the Chart Properties designer.
3. Select a data series from the Data Series listbox.
4. Click on the Symbol dropdown arrow to open up the list of available symbols.
   
5. Select the symbol shape, for example star, to add it to the selected data series.
   
6. Click on the Color dropdown arrow and select Deep Pink.
7. Deselect the Outline checkbox to remove the Deep Pink outline from the star.
8. Increase the size of the star from 5 to 10.
9. Click OK once you are finished modifying the symbol's appearance.