How do you cite from a presentation or poster at a conference?
The specifics of the citation would depend on the citation style you are using. I am most familiar with Chicago style. To cite the presentation you've linked to in Chicago style, I would put:

David Ferreira, et al., "Climate of an Earth-like Aquaplanet: the high-obliquity case and the tidally-locked case" (presentation, Exoclimes 2012, Aspen, CO, January 16–20, 2012), accessed June 8, 2012, http://www.astro.ex.ac.uk/exoclimes/2012/pdf/talks/Day02_Ferreira.pdf.

Following these guidelines: http://owl.english.purdue.edu/owl/resource/717/08/

For APA style, you would only cite a presentation in a reference list if there is a tangible remnant ("recoverable data") of the presentation (e.g., slides posted on a website). The citation would look like:

Ferreira, D., Marshall, J., O'Gorman, P., Seager, S. &amp; Lau, H. (January 2012). Climate of an Earth-Like Aquaplanet: the high-obliquity case and the tidally-locked case. Paper presented at Exoclimes 2012, Aspen, Co. 

For more examples in APA citation, look here: http://citationonline.net/CitationHelp/csg04-manuscripts-apa.htm#53