In unbound mode, you can populate the control with data by generating its content. For generating content, you need to add items either at design time, or at run time.

The following code populates C1MultiSelect control with customer names when the application loads at run time.

Private Sub Window_Loaded(ByVal sender As Object, ByVal e As RoutedEventArgs)
    mselect.Items.Insert(1, "John")
    mselect.Items.Add("Alex")
    mselect.Items.Add("Sean")
End Sub

private void Window_Loaded(object sender, RoutedEventArgs e)
{
    mselect.Items.Insert(1, "John");
    mselect.Items.Add("Alex");
    mselect.Items.Add("Sean");
}