One advice, try e107 from e107.org instead....it's running better and faster then both postnuke and phpnuke on e-smith. But ok, enough about that, you need to say "tar -zvxf nameofthetar.tar" in /home/e-smith/files/primary/
If it says you don't have a permission, then you don't have....you must login as root for this to work. Hope it helps!