Just trying to get into Ubuntu myself and same problem - no Network manager option in the system->administration menu
Also, network-admin doesn't work from a command line, prompts me to install gnome-network-admin.
sudo apt-get install gnome-network-admin
"Couldn't find package gnome-network-admin"
via Synaptic :
gnome-system-tools is installed
lspci | grep Net
0c:00.0 Network controller: Broadcom Corporation BCM4311 802.11b/g WLAN (rev 01)
wlan0 IEEE 802.11bg EESID:""