Within AndroidManifest.xml its not activity but application
<application
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@android:style/Theme.Holo.Light">
<activity
android:name=".MainActivity"
android:label="@string/app_name" >
</activity>
</application>
Give that a try
/Darren