Since this is a compressed archive, you cannot simply drop it into the game folder; you must extract it first. :
Right-click L4o_c3k8e0r0.5.rar and select or Extract to L4o_c3k8e0r0.5/ using a tool like 7-Zip or WinRAR. Identify the File Type : If you see a .vpk file: This is a standard mod file. L4o_c3k8e0r0.5.rar
: Merge these directly into the left4dead2/ directory. If asked to overwrite existing files, ensure you have a backup of your originals. Enable in Game : Launch Left 4 Dead 2. Since this is a compressed archive, you cannot