I know this is a very old post but I just recently got into CNC thanks to a surprise Christmas gift of a Genmitsu 3018 pro. My only available control computer for it is an old Acer c720 chromebook. I’ve got that up and running with crouton and Ubuntu 18.04. I was able to get UGS and Candle working with some effort but Easel stubbornly refused to connect.
Here is what I finally did to get it working, in case someone else runs across this post:
Install the unofficial Easel drivers from here: https://github.com/samyk/easel-driver
Note that if you do not have curl installed you may need to install it first:
sudo apt update && sudo apt upgrade
sudo apt install curl
curl --version
Okay, now you have the unofficial Easel drivers installed. Now run this command in the terminal:
screen -r easel
Now make sure your cnc machine is on, usb plugged in, etc. and log in the Easel website and pull up a project.
If you’re like me, at this point your terminal screen where Easel is running will start saying over and over:
Error Permission Denied, cannot open /dev/ttyUSB0
This is Easel trying to connect to your machine.
To fix this you need to add your user to the dialout group and also give permissions for the usb:
sudo adduser MyUser dialout
sudo chmod a+rw /dev/ttyUSB0
For me, with Easel open in Firefox this instantly changed the carve and jog buttons from blue to green and I was able to now control my machine from Easel for the first time. You may need to log out and back in to Linux if it doesn’t but that is what got it working for me! Hope this helps someone out there as I have spent weeks trying to get this working 
- Glen