I have connected the WMR968 to the iMac with the same RS232/USB connector I used with the PC and installed the drivers but there is no communication between the two.
That's a big red flag.

You will need a Mac driver from the manufacturer for the usb/serial connector.
There are many posts on this around here so I won't bore with the details, but without a modern driver specifically for OS X, you will need another connection method. All of us with the Keyspan adapters (and the driver) have never had a problem. You definitely get what you pay for in these adapters. I believe we have the same setups so don't hesitate to shout out for help should you need it.
...and Welcome to WD Mac!

jmar