In which event are you trying the code? Are you trying the code after binding the RadComboBox control?
If you want to select an item after data is bound, then try the code in DataBound event of RadComboBox.
sender, EventArgs e)
idx = RegionCombo.FindItemIndexByValue(
// (RegionCombo.FindItemByValue("5") as RadComboBoxItem).Selected = true;
Could you provide some more information about the scenario, if this does not help?