For some reason I just encountered this error in Left4Dead 2 – fortunately the fix was fairly straightforward – I just needed to install Left 4 Dead 2 Add-on Support.
Right click on the Steam icon in the system tray and click Library.
Now click on the Tools link.
Look for Left 4 Dead 2 Add-on Support in the list.
If it is not installed then simply Install it. If the install is incomplete, right click it, and then select Delete Local Content and then reinstall it.
Thanks to Goathief on the Steam forums for the fix.
Michael Lane's Technology Blog 
So…. 800 megabytes (according to Steam) of files needed to fix something that yesterday wasn’t broken. Epic fail.
Yeah, it’s pretty annoying – I had 350Mb left to download (I didn’t even know I had downloaded any of it). At least the download speed was pretty good.
Either way it’s alot to download to play the game again …
Better than re-downloading the whole game cause I screwed around with the VPK1 file, silly me, the mods out are pretty crappy anyway
Steam is really pissing me off. This is the second problem I have encountered after only owning it for a few days. This is ridiculous. If they don’t figure out some way of supporting their software better, I may just go buy a console.
I’ve found that issues with Steam are few are far between at least – and I have been using it for quite a while.
Same here, i have had it for 2 days and so far i have encountered 2 different problems.
@ Grant
lol, don’t buy a console, buy starcraft 2 š
Lol.. I tried to play with my friends.. i just putted Add-on support to install few minutes ago and i got the Enforcing thing.. Thanks for telling that it works after its installed :)) My Download speed is 654Kb/s š
this doesnt work for mac platform… says “Left 4 Dead 2 Add On Support is not available for your platform” argh and im trying to get Helm’s Deep Reborn (addon survival map) to run… grrr
is there any way to get this for the mac??? i also want helms deep
I have a Mac (Intel Core Duo 2.0 mHz, 4 Gb RAM, and whatever video and audio cards that Apple installed in its 2008 MacBook aluminum) and I run Helms Deep Reborn with no problems, ever. I usually play with 3 other buddies online, each of them run Windows. I’m not at home right now, so I can’t tell you which version of HD I’m using, but whichever it is, it has worked fine since the day I downloaded it.
I’ve had L4D2 for quite awhile, and I have encountered so many issues I can’t even remember them all. For instance, I got stuck in loading screen for some reason, I got disconnected even if I had internet connection, my computer recieved a bluescreen when playing L4D2, my computer froze (This could’ve been because of the heat problem it had before, but who knows..) and many, many more. I don’t know why L4D2 has so many issues, but I agreed. Steam really needs to look into this, as well as the developers.
Well thermal issues can cause all sorts of problems, but if your machine is stable I’m surprised that you have had so many problems …
just did the most recent update on steam and now i am getting this error message. I cannot update the addons feature in tools as I am also a mac user.
If anyone figures out a work around for mac please let me know.
not available for mac, thanks steam
I don’t get what’s the trouble with this game.
I just played a couple of versus games and when I tried to host one myself, this bug appeared and ruined everything for no reason. Now I cannot play anymore.
Thank you, this worked for me. I was ABOUT to nerdrage.
I do everything as it says, I play 5 to 7 hours and the problem appears again. WTF?
Same problems here… Back to L4D1!!!
sorry i have a problem w L4D2 like this : entereing consistency for this file : scripts/infected/spitter.mld and i have a nonsteam L4D2 can someone help me ???
man, i don’t have the “left 4 dead 2 add-ons support” help me D:
HELP PLEASE!
It doesn’t show up in the tools section please help anybody!!