Have you got it working OK yet?
I'm using a FTDX101D and it works fine for RTTY with MMTTY.
You don't need to use the SignalLink USB with the FTDX101D or MP. You only need to have a USB cable between the radio and your PC.
You should see two additional serial ports and a sound card that are built in to the radio, on your PC after plugging in the USB cable.
The two serial ports are called 'Enhanced' and 'Standard' on the PC device manager. The Enhanced port is used for CAT control and the Standard port for PTT and FSK keying. The sound card is a standard one and if selected in the MMTTY sound card setup menu you should then see the XY and Spectrum scope and decoding being displayed on the screen if you are in RTTY mode.
73 Paul G3WYW