How can I control the location of a dialog when using ShowDialog to display it?

You can set the Form.StartPosition property to FormStartPosition.Manual and then set the Form.Location property to your desired location. When you call ShowDialog the form should show up in the desired location.

MyForm frm = new MyForm();
frm.StartPosition = FormStartPosition.Manual;
frm.Location = new Point(10, 10);
frm.ShowDialog();