Since I still have no icon for plasma-widget-networkmanager even after installing this package, I found wicd in Synaptic Package Manager which helps me set up wireless connections in KDE Plasma Desktop. Here is how:
1. Install kde-full
2. Install wicd
3. Desktop > Main > Add widgets
4. Click on wicd client widget, to install it
6. After reboot, there's a wicd client icon on the taskbar, ready to be configured.