For an unpublished manuscript use the following format:
Author, A. A. (Year, Month Day). Title of manuscript. Unpublished manuscript, Information of electronic archive (if available).
Blackwell, E., & Conrod, P. J. (2003). A five-dimensional measure of drinking motives. Unpublished manuscript, Department of Psychology, University of British Columbia, Vancouver, Canada.
Some unpublished documents fall into the APA category of Archival Documents and Collections (Rule 7.10). Use the following format for these types of information:
Author, A. A. (Year, Month Day). Title of material. [Description of material]. Name of Collection (Call number, Box number, File name or number, etc.). Name of Repository, Location.
This format can be modified as needed. Include as much information as you can that will direct the reader to the information source.
Frank, L.K. (1935, February 4). [Letter to Robert M Ogden]. Rockefeller Archive Center (GEB series 1.3, Box 371, Folder 3877), Tarrytown, NY.