[year] U.S. Federal Census, entry for [given name] [initial] [surname], [locality], [county], [state], [source consulted], [url of source consulted] (accessed [date]).
11. 1880 U.S. Federal Census, entry for William E. Hook, Missoula, Missoula, Montana, Ancestry.com subscription database, http://ancestry.com (accessed April 27, 2012).
Use the actual spelling of the names from the cited record, even they are incorrect. Otherwise, smeone checking the source may not find the record doing a search query.