If your scenario concerns unbound mode, you should set the Value property for each RadComboBoxItem that you add to the RadComboBox.Items collection. Then, you will be able to get the value in the SelectedIndexChanged event handler. I have demonstrated this in the sample project attached.
Regarding the bound mode, when you set the DataSource, you should also set the DisplayMember and ValueMember properties.
If you need additional assistance, do not hesitate to contact me.