Also, look at this thread. someone had the same error
http://www.gog.com/en/forum/the_witcher_2/a_string_of_bugs_launcher_not_working_followed_by_reinstall_refusing_to_try_and

I did all that stuff you told me and now the patching worked and the mod worked, but the dlc still doesn't.I even tryied to go to a new game, but I did not get the items either.

bump

did you check that patch 1.1 is installed?
the dlc need it for them to work

check the readme file, see if it contains patch updates for 1.1
if not, you can download the patch manually here on gog

Where can I find the read me with the version that I have?

it should be where you installed the game

what version do you have btw? retail dvd?, gog?

I have the original GOG version, and I found some readmes there but they dont tell anything about version (at least I did not find).

sounds like you need to install patch 1.1 manually then.
my readme says

The Witcher 2: Assassins of Kings ReadMe file
Update - May 25, 2011

then further down

2) Patch 1.1 Corrections


it makes sense, cause the dlc will only work with patch 1.1

no, you need to download the patch 1.1. the link I posted earlier is only for the dlc files themselves.
to download it. click on "my account "
click on the" witcher 2 box "
a popup will apear, scroll down and click on patch 1.1 to download it.
install that and check the readme file again
yes =D
patch 1.1

Like now I have the patch working (at least I have that stuff in the readme file) but DLC still does not work.And I ran the DLC files again after I patched it.

so nothing appears in game?

Yeah, nothing.....Should I try to download the dlc from the launcher?

no, the dlc in the launcher is broken. they told us not to mess with it, and thats why they released the exe files

it doesnt make any sense. it should work, darn it :P
only thing I can think of is installing the dlc before you had patch 1.1 broke it. other then that im out of suggestions

Do you have any other ideas then?.....