adding the line
<gap:plugin name="com.phonegap.plugins.PushPlugin"/>
in config.xml is the way you add a plugin with the phonegap build service.
When you are doing local builds for cordova or phonegap, you should use the CLI (command line interface) to add plugins.
For the push plugin, either launch
phonegap local plugin add https://github.com/phonegap-build/PushPlugin.git
or
cordova plugin add https://github.com/phonegap-build/PushPlugin.git