I would like to use the Checkboxes property of Rad ComboBox version 2009.3.1314.35 in my application but it is not showing that feature and i am unable to use this in my application.
Can you please provide more details how to use MultiSelect feature with the above version.
Checkbox feature in RadComboBox is added in the version Q2 2011 (2011.2.712) which is not supported in the version you are using.
Another approach is you can use CheckBoxes in ItemTemplate. You can find more about this in the following help documentation. Templates Overview.