If you are using the population data source settings on the control, then you specify the value member. It might help if you just look into how a combo box natively works when using a DataSource as this is really all that we do behind the scenes. We populate based on your PopulationDataSourceSettings property or Enumeration then create an ADO.NET data table that has two fields...Description and Value. The description is what you see that is defined in the Display Member Format String field and the Value is set to the Value Member.All Lists and Combos have a property called SelectedValue which gives you the "selected' value of the control. Likewise if you want to get the display value you can just reference the Text property.
For example:
Dim lnValue As Integer = CType(MyCombo.SelectedValue, Integer)
or
Dim lcDisplayText As String = MyCombo.Text