You need a coaxial S/PDIF cable which is the same type of your interface. There are also optical S/PDIF cables, but not the Virus TI nor the Motu use those.
For the proper type of coaxial cable, read this article on Wikipedia:
S/PDIF
I made custom cables by following those specifications with success. While you may use standard RCA cables (for composite video or consumer audio/video), this is not advisable because you may face issues.
Also, Virus will act as Master AFAIK so you'll need to proper configure the Motu. Anyway, try setting MIDI Sync to External and see if this way the S/PDIF Out of the Virus can be slaved.