Gather information about the session where the poster was presented. This will include the author, the name of the the presentation from the poster, and where the poster was presented. If possible, get the information on the date of the presentation.
Follow the format for citation in the "Publication Manual of the American Psychological Associaton" sections 4.16 D and F. It is as follows: Author. (date). Poster Name. Where Presented. The name of the the poster must be italicized or underlined.
Plug the information you gathered from the poster session into the APA format. For example: James, R. S. (2011, July). Creating a New Apathetic Generation. Poster session presented at the annual meeting of the American Sociological Association, Nashville, TN.