This is basically going to be the same for both the UltraTextEditor and the UltraComboEditor; the following code is for the UltraTextEditor, but simply replace the UltraTextEditor instance with the UltraComboEditor instance and it should work:

private void button1_Click(object sender, EventArgs e)
{           
    EmbeddableUIElementBase embeddableElement = this.ultraTextEditor1.UIElement.GetDescendant(typeof(EmbeddableUIElementBase)) as EmbeddableUIElementBase;
    if(embeddableElement != null)
        this.ultraTextEditor1.Editor.EnterEditMode(embeddableElement);
}

-Matt

VB.Net Version  (according to http://www.developerfusion.com/tools/convert/csharp-to-vb/)

 Private Sub button1_Click(ByVal sender As Object, ByVal e As EventArgs)
    Dim embeddableElement As EmbeddableUIElementBase = TryCast(Me.ultraTextEditor1.UIElement.GetDescendant(GetType(EmbeddableUIElementBase)), EmbeddableUIElementBase)
    If embeddableElement IsNot Nothing Then
        Me.ultraTextEditor1.Editor.EnterEditMode(embeddableElement)
    End If
End Sub