As the U.S. Census doesn't have an individual author, the bibliographic citation for the document should begin by listing the country -- United States -- and the agency behind the work -- Census Bureau. Then comes the city in which the document was printed, as well as the name of publisher and the year of publication. Finally is the medium in which the document is accessed. Government documents are published by the Government Printing Office, which is shortened to "GPO" in MLA style.
A works cited page reference for the 2010 census would read: United States. Census Bureau. Washington: GPO, 2010. Print.
When the information is referenced in the text, readers should be pointed to your main citation through the use of a parenthetical citation. The citation should include only the words "U.S. Census Bureau" and the number of the page where the information can be found, and it should come at the conclusion of the sentence in which the data is mentioned.
An example of a parenthetical citation would read: The demographic shifts were clear in the past decade (U.S. Census Bureau 174).