Basic Format:

U.S. Census Bureau (year data was published). Name of data or report. Retrieved from [URL].

Example:

U.S. Census Bureau (2011). Selected housing charactersitcs, 2007-2011 American Community Survey 5-year estimates. Retrieved from http://factfinder2.census.gov/faces/tableservices/jsf/pages/productview.xhtml?pid=ACS_11_5YR_DP04.

You can see how to cite government documents and many other types of resources in the APA Citation guide: http://libguides.usc.edu/APA-citation-style