Using the manuscript item type, enter the Author as Last, First and then in the Archive field the City, County, State and other location information. Following that, enter the Census title in the Location in Archive field.
Author Last: Mellen
Author First: John
Archive: Cambridge, Massachusetts, page 269, NARA micropublication M252, roll 20
Location in Archive: 1800 U.S. Census
The first citation will look like:
John Mellen (n.d.), 1800 U.S. Census, Cambridge, Massachusetts, page 269, NARA micropublication M252, roll 20.
The subsequent citations will be simply the surname, unfortunately. You can work around this by copy / pasting more of the citation to differentiate between different persons of the same name in the document. In the above example the result would look like
Mellen, 1800 U.S. Census, Cambridge.
Also, the “(n.d.)” field in the first citation is not necessary and can be removed with the possibility of breaking the link to the main data in Zotero. An alternative is to use the persons birth year in the Date field to differentiate similarly named persons.