feed them frozen brine shrimp. i had mine in with a betta too, and i first dropped a chunk of food right infront of the bettas face, then find the frog and drop another chunk of food right over his head, then use my finger to kind of push the chunk down until it started melting and sunk to the bottom and the frog could grab the decending pieces of brine shrimp.
it was rather funny because the frog actually picked on the betta. not enough to cause damage, just chased him around a little.