installing ubuntu on chromebook

I am trying to install ubuntu on my chromebook 15, this has a intell processor.I am in developer mode and have downloaded crouton. Ideally I would like to intall ubuntu on my 500 GB usb drive, When I enter the following commands in the terminal. the following is what I always get.

shell
sudo sh ~/Download/crouton -t xfce
Can't open /home/chronos/users/download/crouton

Let me know if you need any more information. I appreciate any help you can provide