When I try to buy it, the Software Centre just redirects me to the "Ubuntu Software Centre Terms of Service".
What should I do?
OS: Xubuntu 12.04 LTS (TR Language and Time Zone)
You need to scroll down and accept the Terms of Service. You may need to create an account with Ubuntu to install this application. The "price" of $0.00 is so Steam can track how many people are using the program. You can also download it from the Steam website.