Unable to use API devices with microsoft visual studio 2012

Hi I'm using Xamarin with microsoft visual studio 2012. When I tried running the project, the "running devices" don't show up the selected API, ie API_15. I tried use another version of API such as API_7 or API_10, but it's still not work.
I have google this problem but seem like don't have any related information. So, I would like to ask is anybody have face the problem and how to solve this? I attached a picture for clearer understanding of my problem.
Thank you very much first.