When the EN US forums come back up, check this thread for pointers about how to unzip and install addons. There are lots of links to tutorials, including Lunessa's guide to troubleshoot addons on Vista.
http://forums.worldofwarcraft.com/th...43408034&sid=1
Edit to Add: You'll need to right click on the zipped file and extract the contents. Double clicking does NOT decompress files, and therefore the game still can't read them. Windows should have this option automatically.