i'm having that very same problem, with the mx200 ,the spdif has to be connected both in and out,which it is, and then you have to make an audio input somehow in logic and assign it to the correct channel on you interface, not sure how to do this in logic express, i know logic pro has the I/O plugin which makes this very easy but express doesn't have that,
basically i need to setup a hardware effects unit in logic express via spdif