This is a mini howto for OsX (but linux will work similar). For opening a terminal to the USB port of the DevKit you don’t need a serial terminal. screen will do as well.
First you need enough power on your USB port so the DevKit can boot.
I use a BC USB hub as described here (Power from USB + use the usb-serial port as well)
Step 1: Find the USB serial port in your device list
$ ls -lsa /dev/tty.usb*
0 crw-rw-rw- 1 root wheel 18, 12 Jan 8 22:35 /dev/tty.usbmodem1431101
Write down or copy past the name, the name can be different from this example.
Now all you need is screen to connect to the serial device
$ screen /dev/tty.usbmodem1431101 115200 -L
And you have your terminal into the DevKit 
username: purism
password: 123456
Use: Ctrl + A and then Ctrl + K to kill the session.
Hope it helps,
Wxll