Open Terminal and type:
ls -l /dev/ttyACM*
you will get something like:
crw-rw---- 1 root dialout 188, 0 5 apr 23.01 ttyACM0
The «0» at the end of ACM might be a different number, or multiple entries might be returned. The data we need is «dialout» (is the group owner of the file).
Now we just need to add our user to the group:
sudo usermod -a -G dialout $USER
sudo chmod a+rw /dev/ttyACM0
You will need to log out and log in again for this change to take effect.