Q. How do I create an in-text MLA citation for archival materials?

For the in-text citations in MLA, you can use the following format:

[Author/description of item, date/page number (if any)]

For example:
(Alton to Dungan) -- if there's only one letter you're citing by Elizabeth Alton, i.e. a letter from her to the Chancellor of Higher Education.
(Alton to Dungan, 3 March 1969) -- if there's more than one letter from Elizabeth Alton to Ralph Dungan you're citing

(Atlantic City Electric 23) -- Report by AC Electric, page 23

(Atlantic City Press) -- only one article cited in your paper and you don't have the author's name
(Atlantic City Press, 3 March 1969) -- if you're citing multiple articles but don't have the authors' names

Here is a web page that gives a little more information on in-text citations for archival materials:
Archives MLA