If you don't actually own a hl2 game, these are ways you'll have to proceed, and if it still doesn't work you can go to http://www.steampowered.com/nvidia/ or http://www.steampowered.com/ati_offer1a/
if not your quest is fulfilled there you can look to this post
and purchase any of these games now or when they're on sale for 5$ or less...
About Neotokyo still, there are no source SDK base under my tools in steam.
I looked online and find the link steam://install/215
Chose to open with steam but nothing happens. Am I still missing something?