I'm not familiar with the Scarlett products,  but just glanced at the 18i8 manual. 
 
When the ADAT is playing, I would expect the Mix Control meters to be moving on appropriate channels.  I wonder if the clock synchronization needs changing.  In Mix Control, there's an option to set the word clock synchronization to "ADAT".  You might try that setting -- and remember the current clock setting so it can be set back to the original clock source. I'm assuming any Mix Control enabling/configuration for the ADAT inputs has been done too.  
 
Once you get the meters moving in Mix Control,  I would imagine the SONAR setup would go quickly.  

 I've changed sample rate (44.1 to 48) and sync source (from internal to ADAT) within MIX control and nothing happens. I changed Inputs 9 through 16 to ADAT 1 through 8 and I also tried changing the first 8 input to ADAT 1 -8, as well. The tracks and meters in the ADAT are playing along so somethings playing out of the ADAT. Nothing yet in MIX Control or Sonar.   Hmm...    

Brad
 I've changed sample rate (44.1 to 48) and sync source (from internal to ADAT) within MIX control and nothing happens. I changed Inputs 9 through 16 to ADAT 1 through 8 and I also tried changing the first 8 input to ADAT 1 -8, as well. The tracks and meters in the ADAT are playing along so somethings playing out of the ADAT. Nothing yet in MIX Control or Sonar.   Hmm...    

Wish I could help more, but I'm not familiar enough with the Scarlett 18i8.  
 
The only other things I can suggest: double check that the TOSLINK connectors are fully inserted on both the ADAT and 18i8.  Check that the ADAT-XT display shows "CLOCK INT 48K" (if not, use the ADAT-XT clock select button to change it).  Hopefully the optical cable is OK. 
 
Perhaps someone who has used the optical input on the Scarlett 18i8 can be of more help.   Contacting Focusrite might be the next option.  
 
 

Thanks...   One other thing.. Do you think I need to connect tis all to an AD/DA ? I have the Frontier design Tango. I used it with the  old fashioned sound card Dakota back in the day. I didn't go direct to the sound card from the ADAT. I had to go through the  AD/DA Tango 24 converter.  Anyway,   I'll try it thanks.
 
 
 
Brad J Hofrichter
soundcloud.com/bradhofrichter
 

Did you ARM the inputs for recording (in SONAR hit the "R" button on each track lined up to receive the ADAT INPUTS from the Scarlet)?  That may get them working...

O.K I just finally got something going a only few minutes ago. Yes, I armed tracks. Set em' to ADAT IN.  I disconnected the Tango24. Forget that.  Yes, it seems to work.  48 sample rate for one tape and 44.1 sample rate on the other. Sheesh.. what was I doing back then?   The ADAT will let you know what sample rate the tape was recorded at , so I just adjusted MIX control to that. Set the clock to ADAT and it locks. Anyway there seems to be one problem some tracks play and some tracks go in and out or are silent. I get an Err 7 which means I have to clean the heads. Could this be why some tracks go in and out or just won't play? What do I need to clean the heads?  Is there a tape that does this and still available ?   Hey thanks all for helping me here. You've all got me through this mess so far.  Now I know why I scrambled to get a DAW software application on my computer working. ADAT's sound great but what freakin'  pain in the ass.    

Glad to hear you have the digital audio connection working from the ADAT to the 18i8. 
 
The video heads in the ADAT are really sensitive and head cleaning should only be done by someone familiar with the  procedure.  Years ago, I had my ADAT heads cleaned by a pro audio/video repair shop, but the place is long gone.  You do not want to use one of those VHS head-cleaning tapes. 
 
ADAT "Err 7" could explain the audio dropouts.  The ADAT also has an "interpolation light" which will flash when a read error occurs.  It's a star-burst indicator next to the time readout.  If that light is coming on often, it might be an indicator of dirty heads  -or-  bad spots on the tape.  If "Err 7" happens often on all your tapes, perhaps a head cleaning is the next thing to pursue.   
 
For the most reliable results, ADATs required high quality recording media such as Apex,Quantegy, or Alesis' rebranded ADAT tape. 
 
Do you have the ADAT-XT manual?  There's a lot of good info in there.

The manual mentions  going to an authorized Alesis Service center...  Haven't found one yet in my area near Phoenix. 
I'll look for that but no luck yet.
 
 It also talks of a DRY VHS head cleaning cassette.  They recommend 3MVHSHC Black Watch Head cleaner Video cassette or  3M ASD  HC Digital Audio head cleaning cassette.  Since I only need to do this one I might go ahead with that.  I'm nor so sure I'll find it but I'll  look. Thanks for your help.  

Not sure if this applies, But my very first digital? mastering deck was a Hi Fi stereo VHS deck. I bought a top of the line RCA and my first 5 albums I recorded were all mastered to this. It was an excepted format back in the mid 80's. 
But head cleaning was a PITA. You had to take it apart and I would used the same foam swabs and tape head cleaner I used on my Reel to reels and Cassette decks. I would imagine an ADAT uses the same elliptical spinning head system,right? So they are not hard to clean as long as it's possible to get at them. The cassette cleaner carts were for lazy people in those days and they never worked properly. A good engineer always got inside and cleaned manually. 
 
And I would imagine you might have to de-magnatize them too, I never used mine enough to worry about that so I'm fuzzy on this.  Anyhow, just a thought. 

Yes, the ADAT has a spinning video head.  It's my understanding that cleaning a video head requires careful wiping in a specific direction using lint-free swabs (like the foam swabs Cactus mentioned).