Thank you for contacting us.
From what you have provided as a code snippet, it seems that you are trying to select an items which is not in the items collection of the RadCombobox, but having only a reference to such an item.
In order to select the desired item change your code as following:
RadComboBoxItem itm =
(value > 0)
itm = ResrcbActivityType.FindItemByValue(Convert.ToString(value));
I hope this would help you out.
the Telerik team