I may be out of things too much - but I can't see how plugging a video cable into a serial port can help. A serial port and a video receptacle (DVI, VGA, S-VIDEO) are designed to receive specific input. Even if the XBOX is designed to send the video over this cable - there can't be a way the serial port sees this and displays it on your computer - again, unless I am completely out of it.
Now... maybe the cable they are talking about plugs into your normal VGA port. On the EX7 - you get a DVI to VGA connector. The connector looks like a serial port, however, the serial port uses 9 pins (we won't talk about the 25 pin since no-one really uses those these days) and the VGA uses 15 pins. For a novice (sometimes, you can find them in console stores), the interface looks the same and when looking at the back of the computer, the closest thing resembling the VGA is the serial port.
However - once again, I do not believe that it will work. They may be talking about using a real VGA port - and in that case, we are talking about using the DVI to VGA adaptor.
That all said - then the issue becomes - does the VGA port allow incoming signal to be displayed on the LCD. The VGA port is an output port - not an input port. So my answer for this would be no as well.
I believe there are two real solutions for you. The first is getting the tv tuner built into the machine. The second is purchasing a USB tv tuner. Both of these would have appropriate inputs for your signals from the XBOX to work.
I need to stop talking. My head is starting to hurt thinking about this.
Good luck... now I am going to take an asprin.