Ah, the Apple Snail. I was surprised with one a few weeks ago when my husband went to get me an algae eater, but the guy at my LPS said that the snail wouldn't get very big. What a liar!!! This little gal has gone from the size of a dime to the size of a golf ball. I know there isn't a lot of info given at the LPS except to feed them algae wafers....It's all kind of a learning process with these guys. I started by breaking the algae wafers in half and throwing half of one in there every other day. I also leave a floating piece of lettuce in there for her to munch on. I would just watch your snail to see how much it eats. Also, since she is in the tank with one of my bettas, I would float her in a cup when it was feeding time for her, so I could make sure the snail was eating the algae wafers, and not the betta. This way, I could monitor just how much she is eating in an hour or two. Oh yeah, you can google the term "Snail Jello" and you will get plenty of recipes for homemade snail food for your little guy or gal.
www.applesnail.net also has some great information.