MOTU 828mk3 - How to monitor instrument input while creating track?

Hi all, newbie here to MOTU, so I'm sure I'm just doing something stupid. MOTU 828mk3, Windows 7 64 bit, Sonar Producer X1 My question is this: I can't seem to be able to monitor any instruments (keyboard or guitar) that are plugged into the MOTU while I'm recording. In other words, I have my headphones on, I'm playing back a project, I want to record a track of (in this case) Keyboard. I arm the track in Sonar, put on my phones, I have the keyboard (Roland Fantom X-6) plugged into the MOTU using either the SPDIF out from Roland to SPDIF in on the MOTU, or alternatively TRS out from the Roland to the TRS in on the front of the MOTU. When I play the keys, the peak meters in Sonar are registering just fine. If I do a test recording (being unable to hear the keys I'm playing), when I play the project back, the keyboard has been recorded and I can hear it fine, but I can't hear it while I'm recording it. This just doesn't work for me. I need to be able to plug in the Roland to the MOTU, put on my headphones, (which are plugged into the MAIN on the front of the MOTU) hit RECORD and hear the project playing in the phones so I know where to insert my keyboard track and play along. As it is I can hear the project play but not the input from the plugged in instrument. I could do this with the Edirol FA-66, what am I doing wrong here?  Setup on this project is: Phones are routed to MAIN, project files are heard and recorded on MAIN (in or out) 1 &2 in Sonar. As I said, with the old Edirol FA-66, if I plug in a TRS guitar or keyboard to the Left or Right inputs, or connect the keyboard to the MOTU via SPDIF  I can hear it in the phones while I also hear the project playing. Any advice would be very much appreciated.