Are you serious? Did they tell you this on April Fools day by any chance?
The NX8E board already comes with a serial interface. If you purchased just the NX8 (without the "E") then you would need the NX584 serial interface as the NX8 does not have one.
BUT, if you indeed purchased the NX8e, the serial interface is native to that board (I have one myself that I use with HomeSeer via a serial port).
So, unless I'm just not getting something here I don't really know why they told you to purchase the NX584. May want to double check with them (hopefully they take returns).
Also, FYI, the NX8E's serial interface requires a ribbon to DB-9 cable (called something like a DSC05). You will need to pick one of those up as well. If they don't carry one, Martin of Automated Outlet does!