Ask your vet about a laxative before you use it, not all human products are safe for pets. If your dog is still eating normally lack of bowel movement could indicate a twisted intestine which would require surgery.
If the dog is eating regularly and normally goes once a day, I would highly recommend getting them to your local vet for a checkup. Something could have been eaten that is blocking the digestive track.
This is a real concern, and you should make sure your dog has enough water and also enough fiber. However, if it has been this long, you need to see a vet.
If you do not wish to take your dog to the veterinarian, you may perform an enema at home. You should purchase 1 ounce of over-the-counter Fleet enema fluid per 10 lbs body weight at a local store.