To do this you will need to log in to your iNX-Gaming FTP server, using the details provided to you in your welcome email. In Call of Duty 4 you need to create your own 'mod' to run custom maps. The first stage of setting up custom maps on your Call of Duty 4 game server is to create the necessary folders for the maps to run under.
Call of Duty 4 Custom Maps Tutorial Creating the folders and uploading the files to your game server