Easytether is a proprietary application, so you can’t just apt-get it on a chroot’ed crouton environment. Additionally, I am not running an x86 or x64 CPU, so most of these packages will not work.
Crouton mostly comes for Debian, so I downloaded the Ubuntu package. I installed the package ‘alien’. Then unzipped the deb file as follows:
This will 3 tar.xz files, mostly importantly data.tar.xz. Unzip it with the following command:
tar xvfJ data.tar.xz
Now go into the newly created usr/bin directory and execute the easytether-usb command as root and it should work.