Levine, S., & Koltun, V. (2012, June-July). Continuous inverse optimal control with locally optimal examples. Paper presented at the 29th International Conference on Machine Learning, Edinburgh, Scotland. Retrieved from http://arxiv.org/pdf/1206.4617v1.pdf
APA does not address what to do with conference papers found in library databases. We suggest that, following the guidelines given for periodical articles, that you use, "Retrieved from" followed by the URL of either the conference itself or the sponsoring organization, located via Google.
See Style Guide, p. 22.