here is a guide on how to install maps:
First off you want to download the map. be sure that you
save it and not run it. I usally save it to my desktop so it can be found easly.
for vista/xp users only:
Go to my computer click on your hard disk should be: local disk (c). now once you have clicked on this you will see a list of file folders click on the one that says "Program files" once you are in there look for folder named stream. Click on it then look for anather folder named "streammaps" then look for "commen" "left 4 dead" then a foldier named just "left 4 dead" then "maps" now once you have done all this extract your map files right into that folder and you are all set
Hope this helps
Unit