knetworkmanager is for use with KDE. If you're using GNOME, just install NetworManager and NetworkManagerDispacher (which, by the way, are required by knetworkmanager), and the "Network Manager Applet" should be automatically displayed in the top-right panel. Select your AP from the drop-down list, supply the password (which will be stored in you PIM), and you should be connected.
WiFi Manager has never worked well for me, and is, in my opinion, obsolete.
If you need to connect to WPA or WPA2 protected APs, you may also need to install wpa_supplicant.