I would order this one.
http://www.dx.com/p/usb-ttl-converting-board-207841#.VdKHc0r3aK0
The adapter should have the pins labeled. You will only need to use ground, tx and rx.
You can use windows.
Plug the adapter in, it should download the drivers.
Then download a program called putty.
Open up the router. 4 screws under the rubber feet.
Look for 4 pins labeled vcc, ground, tx and rx. Only use ground tx and rx.
Plug 3 wires into the router's serial pins and 3 in the adapter.
Rx from the adapter to tx on the board, tx from the adapter to rx on the board.
Configure putty this way.
Serial line = The COM port your using for serial (check in device manager if need be. )
Speed (baud) = 115200
Data bits = 8
Stop bits = 1
Parity = None
Flow control = None
Have the router connected to the pc with an Ethernet cable to a lan port.
Power up the router. You should see output in putty. If you don't, turn off the router and cross the rx and tx wires.
If you see output, you need to quickly hit the Control and the C key at the same time to stop the boot process.
You should then see a cfe> prompt.
Type, nvram erase, then hit enter.
Have the firmware ready to tftp.
Type: flash -noheader : flash1.trx hit enter then upload the firmware.
You should be running a continuous ping: ping 192.168.1.1 -t
When you do the nvram erase the ping response should be ttl=100.
When the router is done, if it works it should be ttl=64.
