ADAT is a digital protocol (coined by Alesis and the now almost defunct ADAT recorders) that shoves 8 tracks digitally down an optical cable.
It is NOT the same protocol as a standard optical S/PDIF (TosLINK) although the connector is.
Unless your mr8hd has ADAT I/O (I would imagine it doesn't) it's not something you can use. There are interfaces that have ADAT optical I/O for PC/Mac though...