When you have imported your Google_play_services_lib in your project you have to enable your "Copy to WorkSpace" checkbox. So you can find that library project in Eclipse. After that you have to go to
Right click on project -> Go to properties -> Click on Android from left bar -> Add Library -> Click there you will find google_play_service_lib -> Click Apply and ok.
So it will be added as a library project to your project.
Second option is that you can import GoogleAdMobAdsSdk-6.4.1.jar jar file into your libs folder.