In reply to Cadairmanuk: Why didn't the shop assistant show you how?
I believe this will do it for you:
1. Slacken off the cord lock on the blue or red cord on the front of the pack.
2. Post the shaft of the axe through this cord loop.
3. Pull the ice axe attachment bar until it hangs loosely from the pack.
4. Thread the bar through the hole in the head of your axe.
5. Pull the cord ( the one that you've posted the shaft of axe through) until the head of the axe is tight to the pack.
6. Secure the shaft of the axe with the cord lock.