To access the ADAT Optical inputs on the enclosure, you should not select optical S/PDIF for digital format in the main tab in the Hardware Setup.
The input format selection on the on the Digital tab only pertains to the Digital I/O on the card. If you do not select Optical S/PDIF for digital format you should see four pairs of input selections labeled Optical 1-2 (ADAT) and so on. These are the enclosure inputs.
Consequently, if you choose AES/EBU as the input format on the digital tab you should get 8 of each type of input available.