If you are having problems changing lock barrels in the Giro bike carrier you may find the following instructions helpful:
Push in the original key and move the locks to the unlocked position (the key won't come out)
Look at the back of the lock and, using a small screwdriver, push the lock retaining catch inwards so that the lock can slide out. Give it a push with your index finger to help it on its way!
Note that on the control grip of the Giro AF there is a soft rubber bung at the back of the lock. Use the screwdriver to lever this out.
Now put one of the new locks onto the new key and push it into place. Note that the lock retaining catch needs to be lined up between the 2 small studs that separate a chamfered area which will automatically depress the retaining catch as you push it in.
The attached instructions should help.