Welcome to the forums prashanta2!
To make sure nm-applet is included in "Startup Applications", make sure its .desktop file is available in /etc/xdg/autostart directory, and is in default state.
Accordingly, please check the output of -
Does it show "nm-applet.desktop" in the list? If yes, check its contents :
ls -1 /etc/xdg/autostart
For reference, here's its contents from my 12.04 installation -
(Note that the "NoDisplay=" value by default is "true". I changed it manually to make it appear in "Startup Applications").
Comment=Manage your network connections
By the way, you can run the "nm-applet" command with Alt-F2 also, so that you don't have to keep a terminal running in the background -
Alt-F2 > nm-applet > press 'Enter'