The response that you got (Serial<id=0x7fc480396a50, open=True>(port=’/dev/ttyACM0′, baudrate=9600, bytesize=8, parity=’N’, stopbits=1, timeout=None, xonxoff=False, rtscts=False, dsrdtr=False)) indicates that python *is* able to open the serial connection to the EBB if you’re in the correct directory.  If the serial port permissions were the problem, it would have failed at this step.

The Eggbot Control extension would give you a different error (“No module named serial”) if it were not able to load the serial module, so that’s not the root cause either.   

Can you run one more test here, to help identify the root cause? 
Open up the file eggbot.py, and search for the section that starts with
def testSerialPort(…)

   except serial.SerialException:
                        pass

And replace it with:

   except serial.SerialException as inst:
                        inkex.errormsg(‘Serial Communication Errorn’ + str(inst.args))
                        pass

That should indicate the exact error that’s actually occurring.
(This is python, so please be careful to mind the spaces, or you’ll get new syntax errors.)