The fasteners have two ends to them- one looks like a paddle and the other end looks like a spine.
Hold the gun in your right hand and the fasteners in your left hand with the spine towards the gun.
Lower the gun so that you are looking down at the top of the gun and you will see the area (about 1/2" behind the needle) that has the slot for the spine part of the fasteners to fit into.
Align the spine of the fasteners over the opening (the paddles will be sticking out to the left horizontally) and push the rack of fasteners down into the slot. You should hear the fasteners click into place.
You are ready to pull the trigger and start inserting fasteners through your price tags and into your merchandise.