Unfortunately Mac computers don't have Serial ports as standard so you'll probably be using a USB/RS232 Adapter.
If you insert your USB/RS232 adapter into your Mac's USB port and go to "Utilities > System Information App" you'll probably see your USB device listed there in the USB Device Tree and you'll maybe assume your device is correctly installed. Unfortunately that's not always the case. The virtual serial port is often not enabled until you sucessfully install the manufacturer's device drivers.
You may have received a CD with the USB/RS232 device but we'd suggest going to the manufacturer's website to download the most recent drivers.
As mentioned above, the EasyDnc program should install on your Mac without any problems. But device drivers etc from other manufacturer's might be blocked or partially blocked by new security features in OSX causing the serial port installation to fail.
Start easyDnc, click the CNC icon within EasyDnc and go to setup. In the upper right corner of the setup window is a "Ports" drop-down list. Open that list to see if your serial port is listed. If the serial port is correctly installed it may appear in the list with the actual device name. For example a device manufactured by Key_Span would appear as "cu.KeySerial1" whereas devices based on some generic virtual serial ports appear in the list as "cu.Serial1". If you see your device listed then you should be good to go - setup your baud rate and other parameters and use EasyDnc.
But if EasyDnc Setup/Ports only shows a number such as "1" and, below that, an option for Bluetooth, then you can assume your Serial port did not install correctly.
Go to "System Preferences > Security&Privacy > General" and you may see an option waiting for you to 'Allow' installation of the device drivers or extensions thereof. Select the option to 'Allow' and rerun the installation of the device drivers. Restart EasyDnc and go to setup and look again to see if easyDnc can detect the serial port.
If your device still isn't recognized - We wouldn't recommend permanently enabling the option "Security&Privacy > Allow from Anywhere" but for some USB/RS232 devices it may be the only option. As mentioned above, this isn't an EasyDnc problem, it's a problem with the device drivers of the USB/RS232 device and sometimes you may simply need to set the "Allow Anywhere" option just for a couple of minutes while you install/reinstall the USB/RS232 device drivers.
When the device drivers are correctly installed you will see the serial port in the EasyDnc Setup/Ports drop down list.