string fileName = "... .xlsx";
MercatorImporter.ImporterResults ret = MercatorImporter.Importer.Import(MercatorImporter.ImporterFormats.Excel, fileName);
if (ret == null)
{
    MercatorUi.Dialogs.Stop(MercatorUi._Divers.Iif_langue(MercatorUi.Globals.Langue, "Impossible to continue!", "Onmogelijk verder te gaan!", "Impossible de continuer !"));
    return;
}
if (!string.IsNullOrEmpty(ret.LastError))
{
    MercatorUi.Dialogs.Stop(ret.LastError);
    return;
}
DataTable dt = ret.Result;