MercatorWebService20.MercatorWebService client = new MercatorWebService20.MercatorWebService();
MercatorWebService20.SecurityInfo si = new MercatorWebService20.SecurityInfo();
si.SecureKey = ... ;
client.SecurityInfoValue = si;

MercatorWebService20.DataSetMws ds = client.Zselect("select top 1 * from cli");
if (!string.IsNullOrEmpty(ds.LastError))
{
    MessageBox.Show("Zselect : " + ds.LastError);
    return;
}
DataTable dt_cli1 = ds.Value.Tables[0];