Here are a few things to have a look at if you are struggling with getting signal from an external device plugged into the digital input of the iD14 and into your DAW. This assumes that you are having no issues with regular operation of the iD14 and the only problem that you are having is getting signal from the external device.
Are both of the units powered on?
I know I have done this a couple of times before...
Check Preferred Clock Source and status LED in the system panel
If you are using iD14 with a digital device, you will need iD14 to be the slave, so it needs to get its clock signal from the optical input, therefore make sure that you have the preferred clock source set to OPTICAL.
If the Status LED is green then this means that there is no connection problem between the two devices and so the lack of signal is probably due to a routing issue in either of the devices or the DAW. When it is green make sure that the preferred clock source is set to Optical.
If it is amber then the iD14 does detect a clock source present however it does not match that of the computer and DAW session. You need to make sure that they are set to be the same.
If it is red then there is no clock source or signal reaching the iD14 so there is likely to be a problem with the external unit or the optical cable.
Is the digital cable going from the Digital Out of your mic preamp into the Digital In of the iD14 and does the cable look damaged in any way?
Check that the optical cable is indeed wired correctly, it might be that you have accidentally plugged it into a digital input instead of an output on your external device.
Check that the digital output is functioning on the external device by looking inside the port to see if there is a red light (which is the optical signal), if there isn't then there is a problem with that device. Then plug in the optical cable to the digital output and check the other end of the cable has the same red glow, if it doesn't (or if it looks significantly weaker in intensity) then the cable could be damaged so you should try using a different cable.
Is the external device using ADAT or S/PDIF and is the iD14 set to accept it?
Check in the system panel to see whether the digital input is set to ADAT or S/PDIF and make sure it matches the output of the external device.
Is there signal going into your external device?
It would be worth checking that the external device is receiving audio as it should. For example if it is a mic preamp then check using the signal LEDs that each channel is getting a signal into the unit.