FRUS is an edited collection of primary source materials (memos, letters, transcripts, etc).
The Publication Manual of the American Psychological Association does not provide specific guidance on citing the Foreign Relations of the United States. (FRUS). Putting together the citation is a matter of following basic citation rules that make it easy for your readers to go from the parenthetical reference to the entry in your Reference LIst. In addition, your Reference List entry should direct users to exactly where the item is with the collection.
Information about the specific document. Followed by...
series title (FRUS),
the year of the volume (reflecting the period covered),
volume name (if given),
and page number,
the publisher information: Washington, Government Printing Office, year. (Note: Because FRUS volumes are published many years after the periods covered, the date published will be different than the date of the volume.)
Example: A letter written by a diplomat to the Secretary of State. Since you need to make it easy for your reader to locate the Reference from your in-text citation, you may decide to use the author of a memorandum or letter as the first element.
Clark, Lewis. "The Minister-Counselor of Embassy in China (Cark) to the Secretary of State." May 27th, 1949. in Foreign Relations of the United States. 1949, Vol. VIII: The Far East: China, p. 349. Washington: Government Printing Office, 1978.
Alternatively, many FRUS documents are identified in the title by type of document, e.g., letter to, memorandum of, transcript of, etc. In this case, you may wish to use this word as the parenthetical reference to the first element of your Reference List entry (and referencing the individuals by name or title in the body of the text).
In a telephone conversation, the Ambassador to Vietnam ... ("Transcript," October 24th, 1972)
"Transcript of a Telephone Conversation Between the President's Assistant for National Security Affairs (Kissinger) and the Ambassador to Vietnam (Bunker)." October 24th, 1972, in Foreign Relations of the United States. 1969-1976, Vol. IX: Vietnam, p. 294. Washington: Government Printing Office, 2010.