OK here we go and I hope this helps.
Follow the steps exactly (do not alter it) please!!!!!!!!!

Remove all the serial to USB drivers ports you see in the hardware manager.
Then remove all external USB devices and cables connected included the plotter to the PC (except the mouse and keyboard if they are USB).
Restart the PC.
Perform the FTID.exe file for the USB drivers you downloaded.
Now hook the USB from the plotter to the PC and it should mention it has found new hardware and installs the driver automaticly. If not let me know.
Then go to the device manager and check which serial USB comport is used.
Select this port in your software to be the same as mentioned in the manual.
If this does not work you might choose a recovery point in Windows (Vista or XP).
Choose a point before the plotter driver or any other plotter driver was installed.
Remember that some software installed will be deleted and need to be reinstalled again (NO WARRANTY).
Now install the FTDI.exe file again and hook de plotter on and see if it detecs new hardware and installs the USB drives.
Let us know,
Paco