You have to call the method build() after you have finished describing your notification. Check out the Android reference for an example.
Basically, you have to change your code to the following:
Context context = getApplicationContext();
NotificationCompat.Builder builder = new NotificationCompat.Builder(context )
.setSmallIcon(R.drawable.ic_launcher);
Intent intent = new Intent( context, MainActivity.class);
PendingIntent pIntent = PendingIntent.getActivity(context, mID , intent, 0);
builder.setContentIntent(pIntent);
NotificationManager mNotificationManager = (NotificationManager) context.getSystemService(Context.NOTIFICATION_SERVICE);
Notification notif = builder.build();
mNotificationManager.notify(mID, notif);
Note: this code will only allow to show your icon in the notification bar. If you want it to persist there, you will have to use FLAG_ONGOING_EVENT