You have a number of ways you can go now. I'm no USB installation expert. I believe there is a way of finding the USB "signature", that might help find the .inf file. What happens exactly when you install the one it wanted to install? Does the yellow "!" go away? I would let it do its stuff and then see what COM port it reports.
Other options:
- Get (or make) an FTDI cable, and plug that into TxD, RxD, Gnd and +5V. Also connect up RTS to reset via a 0.1 uF capacitor.
- Program via the ICSP header - you could configure your other board to have the ArduinoISP sketch on it
- Reflash the U8 chip - that might make it look like a standard Mega2560 ... and it might not.
- If you have a SD board handy you can use the "hex file uploader" sketch I wrote (link below) - note that the pins on the Uno side are different as it uses MOSI/MISO/SCK for the SD card
- Find the .inf file and get the U8 chip on your Mega to work
- I think there are posts about people who remove the chip from their Uno (or Duemilanove board) and just use that as an "go through" FTDI board
Hex file uploader:
If you are planning to reflash the U8 chip I would first use the same process you did to find the chip signature before, but this time plug into the ICSP header (for the U8 chip) instead. That might help confirm what firmware is on it. Check carefully you get the pins right.
Also before reflashing download the current contents of the chip, just in case.