Actually, this is backwards relative to US English. While job is very often used to describe something you are paid to do, it often times isn't. For example, "my dad gave me a job to do. I have to wash the dog" or "the center's job in basketball is to protect the rim" (note this is true whether he or she is a professional or amateur player). In short, job is often used to describe a chore or specific responsibility even if you're not paid for it.
While the words occupation and employment can be used to describe something other than a professional job situation where you are paid (e.g., he employed his time well), they are primarily used, and this is especially used for the word occupation, to describe a job where you are paid.
So, in summary, job is more flexible and general in its usage than employment, or especially, occupation. With all this said, job is still more commonly used than either employment or occupation when referring to a professional job where you get paid.
I'm pretty sure in a lot of these lessons, if you don't read the hints, you don't get taught some of the words that come up. I don't think I was ever privvy to the word for "sister" in the Family section when I did it (hearing it, that is), so I got confused...I typed "erma"...it was close, LOL...but still got it wrong even though everything else was right. >_>