this is quite normal for labradors not to bark at strangers as they are bred for their placid and friendly temperament. It never bothered us (i hate dog barking) however, i can see why you would want yours to.
One technique to get a dog to bark for strangers at the door is to get someone (a friend or someone he doesnt know too well) to stand outside and ring the doorbell. Then get them to hold a treat through the letterbox so the dog can see it and will start barking. After a few seconds, give him the treat. Repeat this a few times, and he will associate the doorbell with food, and will bark when someone calls (treat or no!).
If you are outside and you want the same reaction, teach him the 'speak command' by holding up a treat, and repeating 'speak' until he barks, then give him the treat.Once he has learnt the word, you can use it outside when being approached by strangers, or other animals etc.
Always remember to keep a few treats in your pocket when you are out - labs will do anything for food! But as a lab owner you will have to accept that they will never be effective guard dogs,and other dogs (and humans) know this.Be prepared for an inevitable dog fight if your lab barks at the wrong dog. And from experience, the lab always comes off worse
Hope this helps