When search online for answers on how the Network Manager doesn't show up in the GNOME notification area one find hundreds of confused forum posts (mostly Ubuntu). There are only two reasons:
  1. Your Network Manager setup is somehow broken
  2. There is no network device to manage
The second case is propably going on in most of the cases. When you check your /etc/network/interfaces and see something like: 
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp
... it cannot work, as Network Manager to manage your connections it needs to look like: 
auto lo
iface lo inet loopback
Restart Network Manager (e.g. "/etc/init.d/network-manager restart") for the nm-applet icon to show up.