wrong password maybe?
Try opening a terminal and run this
Post the output of the terminal if it does not work.
OK, I entered that, it then prompts me for a password, which I entered. It then kind of flicks the VL background for a split second and disappears, and then in terminal it says:
brian:$ vsuper gslapt
sudo: sorry, a password is required to run sudo
I know the password is correct, I am 100% sure. I tested this by following your instructions again, but deliberately entering an incorrect password. It then told me the password was incorrect until I entered the correct one.