The Census has changed access to population data. American FactFinder has been decommissioned and is no longer available. Data are now available at: https://data.census.gov/cedsci/
For the latest population load using the API see, Census Population Using API.
If the state has Townships the process is different. Before downloading city census data navigate to, http://www.census.gov/popest/data/cities/totals/2013/SUB-EST2013-4.html and check if the state is listed on the Townships screen. If it is, use the following steps to download the census.