Cindy
Your ancestor Wm. "Mullenbrook" was using only one of some 14 to 20 different
spellings of this surname. My own MLNBRKs are found under five or six
different spelling variations, both in USA and Germany. Some of these are outright
"misspellings" and others are merely attempts at translating the sounds of the
name into English, or into the local way of spelling in Germany (different
locations had different ways to do it).
Please see my "all names" MLNBRK web site at:
<A HREF="AOL.de | Kostenlose Email, Nachrichten & Wetter, Finanzen , Sport und Star-News auf AOL.de - AOL.com
ALL- MOELLENBROCK MUHLENBRUCH page</A>
I would agree with one of your correspondents who suggested that you try to
get more information on this person from USA records first, before attempting
to find him in the German records, only some of which can be found in the LDS
database. Otherwise you do have a needle in a haystack, despite the fact that
the surname looks a little unusual. It's not.
His first names will be little help as almost every Tom, Dick, and Harry in
northern Germany wears the names Friedrich, Heinrich and Wilhelm. 
Hope this helps.
Kayle Brooks, San Diego CA
Researching: Mellenbrook, Moellenbrock, Muehlenbruch, Mellenbruch (etc.)