Mate, for all your modding needs, head over to the new
Mod Pit for details.
Quick answer is yes, Sandbox is included, and is located on your install disc. Run the installer, and it'll put an exe called Editor in your Bin32/64 folder. Run it, and you'll be in.
Head over to the Mod Pit, follow some of the links, and you'll also find a couple of really useful beginners tutorials. Good stuff there.