Yeah you need it to follow the tutorials in this section, that means getting steam:
steampowered.co mThen any of following packages:
http://store.steampowered.com/app/220/Then go to your tools menu in the steam window. If you use the new steam UI you have to go to libraries-tools then download source sdk base, and source sdk base orange box (if you have it) and then finally get the "source sdk".
Buy it, maybe you can help the development process?