Can't connect via serial GPIO (Rx Tx)

Hello,

i’m trying to connect to the serial port on my RPi3 running Ubuntu Mate 16.04.1 LTS. I am not seeing anything in the serial monitor. On my Mac OSX I’m using ZTerm on Data Rate 115200 Parity -none. Data 8. Stop 1
I am using a SLAB_USB_UART module.

When I tried using serial on my Raspbian Jessie I needed to make an update to my config.txt file like this:

I edited config.txt:
$ sudo nano /boot/config.txtand add the line (at the bottom):
enable_uart=1

I even tried this on my ubuntu mate and this didn’t help.

Is GPIO serial enabled by default? Do I need to try anything else in order to connect to Rpi3 running UM16.04.1 ?