Hi,
Thanks for your reply. After I Google a bit, I found this post from TI Forum: http://e2e.ti.com/support/low_power_rf/f/155/t/191815.aspx
This has solved my issue. I'm running on Windows 7 64-bit and plugging the CC2540 dongle into USB 2.0 port on my laptop solved the problem. The dongle is now detected as TI CC2540 Low-Power RF to USB CDC Serial Port.
Thanks again. Appreciate your comment.