To make a model appear in the 3D view,
three things must be done. First, the "Game Directory" (and/or "Mod Directory") property in Tools / Options / Game Configurations must be set. Second, the models must be unpacked into the proper folder. Third, the entity definition in the FGD must include the studio() helper. This is done one of two ways.
case 1:
@PointClass studio() = entity_name
[
model(studio) : "Model"
]
In this case, the entity initially appears as the flatshaded box we've come to know and love, until a model is specified for its "model" property, at which time the model is then displayed in the 3D view.
case 2:
@PointClass studio("models/some_model.mdl") = entity_name []
In
this case, the model is explicitly defined in the entity properties, and will always appear as this model in Hammer's 3D view.
Note that in either case, the model must be present in the specified folder or it will simply appear as the flatshaded box.