Parsing data for Android L failed.Unsupported major.minor version 51.0
I was facing the same problem on my Mac OSX and for me the problem was resolved by downloading and installing JDK 1.8 from Java SE Development Kit 8 Downloads. You just need to install(or update to) JDK 1.8 and restart the eclipse. That's it! Should work afterwards.
Please check for the following.
You can "solve" it by uninstalling the Android L SDK using the Android SDK Manager.
Not satisfactory.
Edit
I think this answer is effectively suggesting the same thing but without the uninstalling (instead just selecting the version of the API that actually works).