Call of Duty 4: Assets Manager

You can start the Assets Manager, by first launching the CoD4 Compile Tools (found in '[root]/bin/CoD4CompileTools/CoD4CompileTools.exe', it's wise to make a shortcut for it on your desktop), then going to the Applications tab, and clicking the 'Launch Asset Manager' button. You can also directly load it by opening '[root]/bin/asset_manager.exe'.

The Assets Manager for CoD4 simply lets you edit configurations for files that will be included into the Fast File. Whenever you're finished, you usually should press PC Convert -> Current Asset Only to compile the asset you've just made (the asset should be selected). After you're done with the Assets Manager, rebuild the Fast File, move it into the usermaps/[mapname] directory, together with the [mapname].iwd file (if you have one), and run your map.