At last i found the answer for this :Smile:
It's seems that backtrack won't be able to identify our inbuilt wifi adapter when we run backtrack in the virtual machine.
Thus you have to run backtrack in base machine.
To do this:
1.Download backtrack iso file
2.Download unetbootin-windows-585 (
http://sourceforge.net/projects/unetbootin/ ) from this link. This file helps to run your pen drive as a boot-able drive.
3.Mount the backtrack iso file using step 2 software inside the pen drive
4.After step 3 completion, restart your system
5.In bios setting, select usb drive and save
6.Backtrack will get loaded.
7.Now if you use airmon-ng, your wifi adapter will be displayed.
Thanks