<ComboBox x:Name="comboBox1">
    <ComboBoxItem IsSelected="True"></ComboBoxItem>
</ComboBox>

comboBox[0].IsSelected = "True" // this doesn't exit..

comboBox1.SelectedIndex = 0;

comboBoxItem.IsSelected = true;

((ComboBoxItem)cmb.Items[1]).IsSelected = true;

((ComboBoxItem)*testcombo*.Items[3]).IsSelected = true;