Can you Redeem DLC's For Game's You Don't Own? Answer: NO! So stop entering their giveaways.

http://store.steampowered.com/news/5823/

A Steam client update is now available. To apply the update, click the File menu inside of Steam and then select "Check for Steam Client Updates...". The specific changes include:

Features

• Initial client side support for upcoming improved content delivery system (groundwork for future performance/reliability improvements to downloads)
• Added find text in HTML pages ( CTRL+F )
• Added “copy link” option to web browser right click menu
• Added suggested friends from Facebook display in client, if you’ve linked with Facebook
• Added the ability to push screenshots to Facebook after upload
• Additional HTTP API support for Steamworks partners
• Added check so screenshots for unreleased games can only be uploaded as private in the client.
• Added Facebook info when receiving a friend invite.

Bug Fixes

• Improved responsiveness of loading web pages in the client and in-game overlay browser
• Fixed a bug confusing cached HEAD request response with GET requests in Steamworks HTTP interface
• Improved fallback to different servers if first client connection attempt is to a down server
• Fixed a bug not immediately logging off on shutdown in some cases, leaving your state online until a timeout period was passed
• Improved line breaking/wrapping for most Asian languages
• Fixed a rare crash on shutdown in the browser control
• Fixed the store and community sometimes using English in the overlay instead obeying the user's language setting
• Improved Japanese font rendering on the Store and Community for users on Windows Vista or later
• The Steam client now detects if you’re not logged into the Steam Community or Store web pages, and automatically re-authenticates
• Fix a crash occurring if a multiuser chat got enough participants with long enough names
• Ensure user is logged in properly when accessing Steam web sites from the in-game overlay
• Facebook suggested friend UI improvements
• Fixed media playback using external player

• Prevent redeeming gifts of DLC or expansion packs for users who do not own the base game

• Fix the friend aliases dialog in the case where it’s the first time you have used it for a given user.
• The friend aliases dialog now retrieves friend aliases from the server, and now works for chat participants or group members with whom you are not friends.
• Fixed screenshot UI memory leak
• Reduced memory usage after ending a game session in which a large number of screenshots were taken
• Made the screenshot manager prioritize loading screenshots that are visible rather than just loading them from newest to oldest (so you see what you've just scrolled to if you scroll around while loading)
• Fixed steam://openurl and steam://openurl_external links in web pages not only opening the link in the default browser, but also loading a blank black page in the Steam browser.
• Fixed screenshots in D3D10/11 games with B8G8R8A8 or R10G10B10A2 backbuffer formats (Fear 3, Dirt 3)
• Fixed Team Fortress 2 not being playable in certain regions