Mandarins are completely visual hunters. They study their food and wait for it to be looking the other direction, then they strike. You have to convince them that the dead thing in front of them is food. I have not had luck with tobiko but I know others have, I think that may be just the luck of if the individual fish liked eating eggs in the wild. Mine did not (and still don't) recognise it as food.
I have only had luck training them to eat by putting peMysis in a turkey baster (try to only use the small whole shrimp with eyes at first) and suck the water in and out so that the shrimp looks like it is alive and peeking in and out of a hole. When it does pop out of the hole the mandarin usually grabs it. It only takes a few times before they decide that mysis is good even if it is playing dead.
If the fish is too afraid of the baster at first, try live brine in it. The mandarins figure out really quickly that food comes out of that little cave and they will watch it like a hawk. Having them in a confined tank helps a lot with the whole process. They seem pretty nearsighted. Also, use caution to not get a dead brine in the mix. One bite of rotten food and they will be much more cautious to watch for motion before biting and your job will be harder.
The only other thing I would add is that if you have a pair and they are not on frozen mysis (frozen brine doesn't cut it) I would strongly recommend separating them. They will breed even if they are hungry and making the eggs every few days will sap the female's strength so fast she has almost no chance of long term survival (In my opinion)