You have context passed as parameter to onRecieve() method, so just use:
@Override
public void onReceive(Context context, Intent intent) {
//start activity
Intent i = new Intent();
i.setClassName("com.test", "com.test.MainActivity");
i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
context.startActivity(i);
}
It works, of course you have to change package and activity class name to your own.