I want to be sure you have followed this - Instructions for installing addons - viewtopic.php?f=23&t=5158
Go through that guide and see if you can manage to get things working. Let us know if you get stuck and need help.
Be sure to use your mouse to click-and-drag the vpk files into your addons folder, as it says in the guide.
Don't double click the vpk files to install them, it isn't as reliable as it should be.
Your 'Operating System' (OS) means the platform that runs your whole computer, you should see the name when the computer starts or on your desktop. It's probably Windows 7, Windows Vista, Windows XP, Linux etc. If you have a Mac it would be OSX or similar.
Try that guide and let us know if you managed to get things working.