Open up Cod4\bin\asset_manager.exe. You will see several buttons in the top left corner, along with a list. Scroll down to 'xmodel', and click 'New Entry'. You now need to enter the name of the model that you will use to access it ingame, and in Radiant. It can't have any spaces, and try not to make it too generic, pick something which will definitely be unique, e.g. db_laser_cannon. You will see a list of options for your model appear on the right hand side now, select 'animated' from the first drop-down menu. In the 'filename' field, click the browse (three dots) button to the right. Use this to locate and open your xmodel_export file that you made before.
Save your Asset Manager file in Cod4\model_export. Anywhere else and it won't convert, but it won't have any errors.
Click on your entry in xmodel. Go to PC Convert>Current Asset Only, and if it works, you should see 'Conversion done, hit key to continue.

You also need to conver the images that are associated with the model to COD4.

Instead of going to "xmodel" go to "material" in the list.

Here is the tutorial:
http://wiki.modsrepository.com/index.php/Call_of_Duty_4:_Models_imports