I have the original C8 (1992) connected to Pro Plus 6.4.3 on Macbook Pro OSX 10.8.3.
What you will need to know is the following:
My Serial-to-USB driver is from The Source: Nexxtech brand name.
I installed the USB-to-Serial driver from this company: Prolific.
Sign in with "guest" "guest"
Download the driver.
Or download the driver right from this email. I have attached it in my reply.
Make sure you are running the latest version of Starry Night (6.4.3 at time of writing).