I've loaded tuneecu on a few machines and you shouldn't have to assign a com port. You can use any open USB slot. The drivers provide the interface to the program. When the drivers are loaded correctly and you plug it in to a USB port, the driver will tell the program where the cable is.And I'm still stuck -- Toshiba Satellite laptop -- thought I would nail it with the "Show hidden devices" -- and no hidden devices are shown.
I did note that that the FTDI driver allows one to assign a COM port, and when I did so, I see a message that "COM 3 is taken". So when I launch Tune ECU, it's locked on COM 3, but so far, I've found nothing in the operating system to show me COM 3, much less manipulate it.
Did you load the driver under USB serial bus controllers as well as under ports (com & LPT)?