So during the installation process you get that error? The installer replaces the old map with the new one. Maybe -for some reason- your computer prevents you from doing that (replacing files). In case you have installed 911: First Responders into the Program Files folder, UAC can make it harder to make changes to that folder. You can also try to delete your temporary internet files, cookies & cache and then try to redownload the mod. The mod comes in a .zip file, it's possible something went wrong during extracting that file. Hoppah