I followed the instructions on the guide
http://elinux.org/BeagleBoardUbuntu#Demo_Image for newer kernel.
I have also installed GUI for demo . But in the system lots of applet
does not exist. i.e terminal or network connections. How can I
install other applications? No synaptic package manager also.
Thanks
Can you please provide a screenshot of "applet does not exist"...
I'm 75% certain your seeing what happens when gnome decides to be
default in gdm when your install xfce..
So when it firsts boots up to the gdm/login screen.. Click your user
name, now look down.. (this is from memory so please interrupt what i
say, with what you actually see..) There should be 3 clickable down
arrows's.. The one furthest to the right from the center should say
"Ubuntu Desktop something" change it to "xfce Desktop"
If it's not that, take a photo of the desktop so we can have a chance.. 
Regards,