CoD Radiant is the most used mapping tool for Call of Duty.
To install CoD Radiant:
Open up CoDTools.exe
Read the License agreement and agree with it.
Make sure that the directory is pointing to where Call of Duty is installed.
Create a shortcut on your desktop.
Go to your callofduty/main/maps folder and create a new folder. Rename it mp.
You should now be ready to run CoD Radiant for the first time.