Firstly you will need to download pakscape, now open Call Of Duty/Main/pak0.pk3 & go into the xmodel/ folder. Inside this folder are all the world models available to you by default.

Look for a model you wish to use, in this case we will use the tree_pine_mid model.



Now open radiant & in your XY Top view, right click & go to misc - model



Select this & a file browser will pop up, close this. The entity tab should now be open on your misc/model entity.

Into Key: enter "model"
Into Value: enter "xmodelyourmodelpath"

So our entity tab should look like this:



Press enter & your model is now ready for use. If you wish to re-scale your model enter:

Into Key: enter "modelscale"
Into Value: enter "your_new_scale"

So, for example if you wanted it half the size of its normal size, your entity tab would look like this:



Press escape twice to close the entity tab & deselect the model & enjoy.



Before running your map you will need to add the following script into your .gsc file to preache your model (you need to do this with all your models)

precacheModel("xmodel/modelpath");

So our .gsc should look roughly like this:



You can download your own .chm localised xModel browser HERE