Greg,
I had responded to you back in Jan 2002 when you posted this; I only had a Nicholas Theissen (Jr. and Sr) born 1842 and 1807, resepectively, in Manderfeld.
I recently have received more info (and cousins in both the US and Belgium), but the email address I had for you didn't work.

1-Nickolas Theissen (1730-)
sp: Luzia KNODT (-)
2-Leon Theissen (1762-5 May 1844)
sp: Anna Maria JUSTEN (-)
3-Anna Maria Theissen (1801-)
sp: Michel KINGEN (-)
3-Johann Nikolaus Theissen (14 Dec 1804-)
sp: Gertrud HÄMMERLING (23 Feb 1808-22 Nov 1871)
4-Peter Theissen (12 Aug 1829-)
4-Leonard Theissen (11 Apr 1831-)
4-Anna Maria Theissen (21 Mar 1833-12 Apr 1903)
sp: Quiren ARENS (5 Feb 1831-)
4-Gertrud Theissen (6 Apr 1835-)
4-Maria Magdalena Theissen (25 Jan 1838-22 Mar 1842)
4-Heinrich (Henry) Theissen (16 Sep 1840-4 Oct 1930)
sp: Louisa MAILEUTNER (Aug 1852-14 Apr 1890)
sp: Mary BRAUN (BROWN) (2 Sep 1867-Nov 1896)
4-Mathias Theissen (20 Jul 1843-1 Jun 1935)
sp: Anna Katherine ... (26 Oct 1853-17 Oct 1908)
4-Clara Theissen (26 Jul 1847-)
sp: DEEKENS (-)
4-Hubert Theissen (20 Nov 1849-1872)
3-Nicolaus J. THEISSEN (17 Aug 1807-6 Feb 1896)
sp: Margaretha HENNES (22 Nov 1801-7 Oct 1858)
(daughter of Josef HENNES and Anna K DAHM)
4-Johann Nikolaus Theissen (4 Nov 1831-)
4-Leonard Theissen (27 Oct 1833-1 Feb 1847)
4-Michel Theissen (12 Mar 1836-5 Feb 1837)
4-Anna Maria Theissen (11 May 1838-)
4-Nicholas Johannes Theissen (13 Jul 1842-27 Feb 1896)
sp: Anna K. ECKSTEIN (11 Feb 1845-22 Jan 1934)
4-Nikolaus Theissen (1 Sep 1845-)


other descendant names include SEIFERT, LAMBERTZ, PROES, MOLTER, LANGER, ZIEGLER, ECKSTEIN....

Let me know if this helps with a connection.
Pete Hanson