AdRequest.Builder cannot be resolved to a type

I'm incorporating AdMob into my app. I've followed the steps in the developers page. However AdRequest.Builder() is being underlined with red and it says:

AdRequest cannot be resolved to a type

and

AdRequest.Builder cannot be resolved to a type.

What could be the problem?

import com.google.ads.AdRequest;
import com.google.ads.AdView;


public class FireRoomActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_fire_room);

        // Look up the AdView as a resource and load a request.
        AdView adView = (AdView)this.findViewById(R.id.adView);
        AdRequest adRequest = new AdRequest.Builder().build();   
        adView.loadAd(adRequest);
    }

In xml I've shown admob as such:

<com.google.android.gms.ads.AdView android:id="@+id/adView"
                     android:layout_width="wrap_content"
                     android:layout_height="wrap_content"
                     ads:adUnitId="bla bla"
                     ads:adSize="BANNER"/>