Short answer: (2010, August)
POSTER SESSION [Example]: (The part in red is word for word from the APA Manual 5th ed.)
Ruby, J. & Fulton, C. (1993, June). Beyond redlining: Editing software that works. Poster session presented at the annual meeting of the Society for Scholarly Publishing, Washington, DC.
Note:
1. Year and Month are from the Year/Month it is presented; in other words - when the conference takes/took place.
2. City and State are from where it takes/took place that year (e.g., Boston, MA).
3. I have seen people specify the meeting number (e.g., at the 21st annual meeting...).
4. UNTIL you present - specify "accepted for presentation" in place of "presented". [Have seen this done with "submitted to / submitted for presentation to" as well if it hasn't been officially accepted.]