PEI_3721,
as far as I know, the Manual is currently available only on the ICAO-NET intranet (I do not have access to it). This is the status according to the minutes of the ICAO AIG Meeting in October 2008:
Originally Posted by http://www.icao.int/AIGdiv08/docs/AIGdiv08_WP15_en.pdf
2.3 Manual of Aircraft Accident and Incident Investigation (Doc 9756)
[...]
2.3.2 The current status of the four parts is as follows:
a) Part I was published in 2000;
b) Parts II and III were published on the ICAO-NET in the English language this year and should be made available in “hard copy” in the near future; and
c) Part IV was published in 2003.
Yes, you are right. The Circular has been included verbatim in "ICAO Human Factors Training Manual Doc 9683-AN/950" as chapter 4.