How to work with a 3D printer in Ubuntu?

Question

ubuntu

I connect a Prism 3D printer (it's based on the Arduino SA Mega 2560 R3 CDC ACM) to Ubuntu via USB and I have a /dev/ttyACM0 device, but no one can work with it, neither Pronterface nor Cura.
The error in Pronterface is this:

"Connecting...
Got rubbish reply from /dev/ttyACM0 at baudrate 9600:
Maybe a bad baudrate?"

I changed the connection speed to the lowest, there is no effect. Doesn't connect.
Thinking it was a matter of permissions, I tried 1.) sudo, 2.) added the user to the dialout group, 3.) edited sudoedit /etc/udev/rules.d/50-myusb.rules

KERNEL=="ttyUSB[0-9]*",MODE="0666"
KERNEL=="ttyACM[0-9]*",MODE="0666"

but everything is still the same.
What could be the problem?