Is there anyone in your area that have horses that are eating frozen if so take them to that person and place them in the same tank,seahorses learn to eat frozen by watching others do it,another way just go to your local petstore and ask for ghost shrimp they are expensive as a food but it will fatten up your horses fast, you can also force feed them there should be some info for your problem here
http://forum.seahorse.org/index.php?