You have to buy Sage Instant. You get a perpetual licence, which cost me about £114 when I bought it at amazon.co.uk. The version I bought was downloaded rather than being provided on a CD. When I need to download again later, I log into my amazon account to download it from ny "Software Library." Sage provide you with an activation code to start the software off.
The £114 was a one-off payment. There isn't any monthly or annual subscription to pay after that.
Having said that, I have just read that Sage Instant 2014, the one used in the ICB material, was discontinued last April. You would need to buy the replacement, which is called Sage 50 Accounts Essentials. The price is £118.80. Here is the URL:
The ICB strongly recommend you do the mock exams. I very much agree with them, especially for this exam. It's yet more money for you to pay but it will help you to pass.
Finally, you will need this software for this exam at Level II and also the first exam at Level III. I haven't seen any exams after those that require you to use a computer package.