Hi, Patrick.Welcome aboard!Nice to make a new connection.
The page you get when you follow the aforementioned link is the front page for the database.You either need to enter a name into the box to do a search, or click on one of the letters of the alphabet to get all surnames beginning with that letter.
The only William Flandreau (b.1911) I have is from the Social Security Death Index:
WILLIAM FLANDREAU SSN 108-14-8364 issued in New York Born 26 Jun 1911 Died Jan 1996 last known residence 12583 (Tivoli, Dutchess, NY)
I have speculated that he might be the son of Ervin Pye Flandreau of New Rochelle, but that seems less likely if he's your William.Ervin's son William was one of two boys and at least in 1939 they were living in New Rochelle.
Feel free to send me some details of your branch of the family; I'd be happy to add them to the database.Only non-living members appear in the online version.
As for Flandreau, South Dakota, 99% of the links I used to find online for Flandreau were really links to something or someone found in that town in Moody County.The town is (mis)named for Charles Eugene Flandrau, a pioneer politician and lawyer of Minnesota.You'll find him in the database as well...
Look forward to hearing from you.Thanks for posting.I wish all the researchers would.:-)