True, the SK website is pretty much useful only for booking lowest economy price. To get a c-class quote you must search flights by schedule as opposed to price. Then in the price drop down try different price options. When you have choosed a specific price and routing click the fare rules and take Properties on that pop-up page (context click anywhere on the page).
By scrolling down one of the info fields there you will see the booking class in question. To find out exact rules for a specific reservation you actually need to call SK. Try the LH or, probably even better, the OS website instead to gain more control.