I upgraded my 2011 Mac Book Pro from OSX Mavericks to OSX Yosemite a few weeks ago. I found that when creating new Xamarin Forms project, I was getting a compilation error in the Android project.
The error I was getting was the following:
If you download the Java update, the problem still manifests itself. To solve this problem download the Java Update via the Apple Website.
I downloaded the update in this link is http://support.apple.com/kb/DL1572, and it solved my problem. I hope that this helps someone out.