You will most probably have to start with live food. I got one to eat blackworms but refused to anything else; even brine shrimp. They may be acclimated to frozen especially if other fish can "show the way." Some never adapt at all and die of starvation. This is a paticularly difficult fish to acclimate to captivity. You may try pushing food into cracks as this is similar to their natural feeding habits. Longnose butterflies can be acclimated in this manner as well.
I've never had one myself, but a couple of friends of mine have. They've had good luck feeding frozen steamer clams. Basically, freeze a live steamer clam and then put it in the tank. It will naturally open and the butterfly will go to it. HTH