You need to have the Unity android support jar in your classpath. You can find that jar file at /Applications/Unity/Contents/PlaybackEngines/AndroidPlayer/bin/classes.jar.
Or, to save your valuable time. there are inexpensive but good plugins provided at http://www.neatplug.com.
Hope this helps.
EDIT: We now have a trial version for those who need Admob in their projects. The package works seamlessly on both iOS and Android platforms, all you gotta do is a drag & drop.