Complete your first trade before 30 September 2012 to receive a $50 reward deposited to your linked transaction account.
Complete three trades before 30 September 2012 to receive a total reward of $100 deposited to your linked transaction account.
Trade fees from $15.95 a trade.
A pretty good deal for people who trade shares on a regular basis. The cheapest brokerage is $15.95, though, so obviously if you just signed up to get the money, and wouldn't be trading otherwise, it becomes less desirable. I use CMC markets at the moment, which costs $9.90 per trade.
In theory you could execute all of your trades in a single day on the same stock (buy,sell,buy,sell) and assuming you were able to make all of the trade at roughly the same price, you would get $100-4x$15.95=$36. But it might not be worth the effort due to having to then to do capital gains calculations at tax time.
Basically, don't take up share trading just to get the $100, you might end up worse off unless you know what you are doing.