The blue mini-USB console cable that is included with the IR829 (and other 800 series devices) connects the mini-USB console port, under the screwed down plate, to a USB port on your Mac.
To use the "screen" command on OSX El Capitan 10.11 to connect to the console on the IR829, you need install a new set of CP210x USB to UART Bridge VCP Drivers. Then you can:
screen /dev/cu.SLAB_USBtoUART4 9600
Will mention that the Serial application is very good, it does cost $30, but it just works (TM). Also see The specified item was not found..
When using Serial you will need to open (Command-o) a terminal window, whereupon you will be presented with an "Open Port" window, which will allow you to select one of, in my case, two USB to UART bridges. In a case, it was #2 that worked. When then terminal window opens, you will need to open the settings (Command-;) and set the "Terminal Settings->Configuration" option to be "Cisco".
Also see The specified item was not found., and .
But the command screen /dev/cu.SLAB_USBtoUART4 9600 can be different. in another case it was screen /dev/cu.SLAB_USBtoUART5 9600
Cisco has the Hardware Installation Guide for IR8x9. To access the console, you may simple refer to the guide: Cisco 809 Industrial Integrated Services Router Hardware Installation Guide - Connecting the Router [Cisco 800 Series I… or Cisco IR829 Industrial Integrated Services Router Hardware Installation Guide - Connecting the Router [Cisco 800 Series …