I suggest you read the paragraph "The Message Key" here . When I had read it and repeated it slowly to myself 25 times or so, I finally managed to understand what I had missed.
We spent about 2 hours on it when we got the coin and that was the part we were missing as well. I'm sure that's where a lot of people get stuck. Just remember, everything you need to know is on the coin!!
I still don't get it. Pls help me