Download the Phonegap plugin by cranberrygame. you need the "Phonegap Game: show leaderboard and achievements " one, It works with intel xdk - crosswalk. I don't think the google play plugin that came with C2 works with crosswalk.
Note: The plugin will not work in the web browser and you will a need Google developer account.
1. Setup up the leaderboard and achievements in the google play developer console.
2. Get your App ID - from Google
3. Install and add the phonegapGame plugin to your game -
4 under the plugin properties add the app ID.
5. On the Event sheet add Events/actions for login, submit score, show leaderboads, etc..
6. Export your game
7. Open the export folder
8. open intelxdk.congfig.additions.xml in notepad
9. change the the value to your google app id , on this line <intelxdk:param intelxdk:name="APP_ID" intelxdk:value="Your Google APP ID" />
Now you can import it in to XDK and export a APK.