it should be found here (if not you may have to re download it)
and you should add this path to Additional libraries from "Tools > Configure pathes"
you need also to add this in the top of your main activity
#Region Project Attributes
#AdditionalRes: C:\Android\android-sdk-windows\extras\google\google_play_services\libproject\google-play-services_lib\res, com.google.android.gms