You have to download the US client here (14 days free trial) :
http://www.darkageofcamelot.com/download/
A faster way is to copy your DaoC folder to a new Catacombs US one and then edit the update.dat file in the new folder. Open it in notepad and replace the text there with :
Mythic Patch
patch.daoc.net:1280/updates/catacombs/mpatch.dat
patch.daoc.net:1280/updates/catacombs/mpatch.dat
Then run the camelot.exe : it will patch US language and version.
Here is a cd key you can enter :
HQPW1QA-W8IMQ1B-M71E7DJ-BYUNQDD-MA7TAWY
(more are available on the Camelot Herald download section)