Do you have another box that you can test your setup with, i.e. flash an image using dreamup.
Have you used the null modem cable before, do you know it works ok?
Chances are if it's new it might be a serial cable or rs232 cable. These look the same as a null modem cable but the pinout is different and those 2 cables don't work, it needs to be a null modem cable.
You can get them on ebay for £2.