In order to use Maps Android API, you have to assign a API Key in AndroidManifest.xml.
<meta-data android:name="com.google.android.maps.v2.API_KEY" android:value="your API Key here"/>
You have to assign Debug API Key in testing, and Release API Key when release APK. Just insert the correct API Key, clean and rebuild your project should be OK. It's suggested to un-install your old app in device before re-load.
A simple example using Google Maps Android API v2, step by step.