rightclick in a 2D window, choose misc --> misc_model (or misc_gamemodel) .
In the browser window that pops up, choose your model (which should be somewhere in “etmain/models/mapobjects/grenade/grenade.md3” (just an example, but don’t have it on your desktop or smth, it needs to be in “models/…”.
Click OK
Open the entity window (N) and change the path of the model (which is now C:\Program Files\Wolfen…" to “models\mapobjects…” (and insert your path)
Make sure that the path starts at “models” instead of at “C:”
I guess that’s all…