At some point in the game you need to open a box using a code that comes with the original packaging of the game. In ODG that code is hidden, written on the back of the front cover. You actually need to remove the cover from the DVD case to solve the puzzle! The clues can lead you there if you can think "outside the box", but there's also Martin's notes that make things more clear. In a nutshell, you need to solve a puzzle outside the game! CS does not come in a DVD case, and there was actually no place to hide the code, so it just comes written on a piece of paper.

On a side-note, all walkthroughs posted on the internet that mention the code itself should have it removed, cause this way, this clever, old Sierra-like copy protection is rendered useless.