Modding Call of Duty 4 is not as easy as modding other versions, but here are some tips to get you started if you never done it before.
First off, in order to get into the IWD files, you simply need to rename them to have a .ZIP extension, and open them in WinRAR, WinACE or 7zip. Alternatively, I found that associating all of the files to WinRAR, then opening the Main folder in WinRAR, I was able to select all of the IWD files and extract them to a new location so I could play with them.
In order to open the IWI files that you get after extracting the IWD files, you can convert them to DDS by using this IWI to DDS converter.
The cfg files that you have extracted are plain text and easy to edit.
Once you are done modding, you can repack the files as a ZIP file – just make a new archive, and specify the name to be the same as before, for example: C:\Program Files\Activision\Call of Duty 4 – Modern Warfare\main\iw_00.iwd would be “iw_00.zip”. Once the new archive is made, change the extension to IWD, the reload the Call of Duty 4 server.