I don't know about how to set that trap, but good luck getting that stink off your lab. I don't know what it is, but atleast once a month one of my female labs comes home stinking of skunk. The last time I found her on the sofa on the deck tearing the skunk to pieces. It took forever to get the smell out of those cushions.
We used this recipe on our other dog when he got sprayed last fall. Used it again last night. It works well.
What you need
Next go to a drug store and get the following, if needed:
1 quart (or liter) of 3% Hydrogen Peroxide, H2O2.
Use fresh (unopened) hydrogen peroxide (H2O2). Hydrogen peroxide eventually turns into water (H2O).
1/4 cup (50 ml.) of Baking Soda
1 teaspoon (5 ml.) of Liquid Soap
1 pair of plastic or latex gloves
Bathe the dog outside or, if it's too dark or cold, in the bathroom with the door closed and window opened.
Combine the ingredients in an open container (do not store in a sealed bottle--it will explode).
Using gloves, wash your dog with lukewarm water and the mixture while the mixture is bubbling. Work the mixture well into the fur.
Be sure to concentrate on the area that was sprayed.
Keep mixture away from your dog's face and eyes (it's a harsh solution). (If your dog has been sprayed in the face, try Tricotine Liquid Douche Concentrate or any over-the-counter douche.)
Let the mixture stand for 10 minutes or so before rinsing off.
Rinse the dog with lukewarm tap water. Don't wash the mixture into your dog's eyes (use a washcloth to cover the eyes if you're rinsing the head).
After bathing, check your dog's eyes. If they are red and watering, your dog may have taken a direct hit in the face. Skunk spray won't blind the dog, but it's very painful. Contact a vet.
Borrowed from http://dan.drydog.com/patsyann/skunk.html