This question already has an answer here:
I've just finished downloading Ubuntu for my C710 and rather than it being in Ubuntu it is still Chrome OS.
How do I install Ubuntu on a Chromebook?
What Have you done after downloading the Ubuntu for your c710.
Maybe you have to make a bootable USB-Stick and boot from it and install ubuntu. How to make a bootable USB-Stick: How do you make USB bootable on chromebook
Hope this helps.
From the page:
It can be done, and is only logical, as Chrome OS is based on Linux, like Ubuntu.
What it looks like your problem is you didn't set up dual-booting correctly, as Chromebooks work differently than regular computers for booting.
If you want to dual-boot, look here: http://liliputing.com/2012/11/how-to-install-ubuntu-12-04-on-the-199-acer-c7-chromebook.html. It gives instructions that are aimed at the Acer C7 using ChrUbuntu.
Should solve your problem relatively ASAP :D!
Best of Luck, will be following this.
BTW, if you get it working, could you post something within the post or something like a Community Wiki with what you did? That would probably be greatly appreciated in the community!