Where are the electric pieces for 42029?
These pieces are not included in the set. Motorizing this set requires the addition of a Power Functions motor as shown here:



You'll need the Power Functions Motor Set (8293) if you'd like to motorize your truck. This can be ordered from shop.lego.com or other online retailers. It's usually hard to find in stores, but if you have a LEGO store nearby, they will likely carry it.