It seems you have not downloaded the correct package. What you have is a folder called crouton and what you need is a script-file named crouton.
You should change the name of the existing crouton folder or move it to a diferent directory before going further as you won't be able to have two items named crouton in the same ~/Downloads folder.
After you have renamed the crouton folder, download the crouton script file from here (make sure to download it to your ~/Downloads folder): https://goo.gl/fd3zc
Then, run those commands:
sudo sh ~/Downloads/crouton -t xfce
and be patient and "answer the prompts like a good person" . . .
Now, you can go straight into xfce by using the following command:
sudo enter-chroot startxfce4
or, you can run it as a "special shortcut" with this instead:
click here for more info