If you don't know the address in or near 1940 but know the Enumeration District from the 1930 US Federal Census, use Steve Morse's 1940 Census in One Step Website to convert the 1930 Enumeration District to the 1940 District. Remember: the person/family may have changed residence between 1930 and 1940 censuses.
If you have not found the person/family in the 1930 census and do not have any address information other than city and state, search for addresses in:
United States, World War II Draft Registration Cards, 1942 ~ Men, age 45-64 (born 1877-1897), April 27, 1942
Steve Morse : How To Access the 1940 Census Until a name index is created for the 1940 census the only way to use the census is by Enumeration Districts (ED).
Steve Morse has prepared a Tutorial Quiz, which suggests a number of search strategies and second opinions for finding ED numbers. The quiz takes you through a series of questions to determine your particular situation and then suggests your best strategy for using the various tools.