"Go to the 'Projects' tab (in the top-left)
Under 'CORDOVA 3.X HYBRID MOBILE APP SETTINGS', click 'Plugins and Permissions'
Click 'Third Party Plugins'
Click 'Get Plugin from the Web'
Enter the following details:
Name: AdMob Plugin
Plugin ID: com.cranberrygame.phonegap.plugin.ad.admob
Check the 'Plugin is located in the Apache Cordova Plugins Registry' box
Then click 'Import'."
This is quoted from the manual