Using latest version of MS3d:
You have to save the model into its own file. (in this case call the file 'zombie.'
Then go into MS3d and go to tool -> Kratisto's Half-Life model Decompiler v1.2, leave all the fields checked and browse for the file zombie.mdl in the zombie folder. (Don't use the MS3d decompiler cos it screws with the animations.)
Next you need to import the smd. files. To edit the model the smd you need will usually have the word 'ref' or 'reference' in the title. (e.g. zombie_reference) Double click the file and import the mesh/trianges and bones/skeleton.
If you want to make the head and arms more human then your best bet is to rip them straight from a scientist or barney model. Do do this you'll have to do the same again. Make a scientist folder and decompile the model.
Then you have to add the scientist model to the screen by importing the relevant smd. but only import the triangles. Then you'll have to select the groups/materials of the scientist one by one so you can move him away from the zombie's mesh (This means using the high definition models makes it much more simple.)
Next you will need to select the zombies arms and head and delete them and delete everything on the scientist mesh except for the arms and head. Then position the new arms and head onto the zombie's skeleton and then comes the complicated bit. Welding the parts to the skeleton and making it look right.
You have to select the bone first for example 'bip01_larm1' which will be the top of the left arm. In this case you have to select the entire of the left arm. (basically you have to select everything from the selected bone down). Then the forearm (possibly 'bip01_larm2') where you will have to select the forearm and hand and then the hand. Then you'll have to do the same on the right arm and the head.
Thats the simplest way I can explain it.
Actually, at first just to get the hang of things, your better off simply editing a few models like the scientist. Make the coat shorter or make the nose bigger or just sill things like that.