Things that matter for this:
#1-the session sample rate. the lightpipe in ADAT mode only operates at 44.1 and 48K(so the higher sample rates will not work thru ADAT optical).
#2-Hardware Setup>Optical Format; is it set to ADAT or SPDIF?
Assuming the session is 44.1 or 48K and Optical Format is set to ADAT, it might be as simple as hitting the DEFAULT button(Setup>IO>Input tab), or simply adding the paths and connecting via the grid