To feed a creature, you have to go in "My Game" => "My creatures" to see your creatures, then click on a creature to go on its profile. There, it'll have some icons under the creature, and one of them is written "feed". Only click on the button to feed your creature. That's it! If you don't have food, just click on the "buy" link and you'll be able to buy some new food for your creatures!
You can only feed them one by one.
Have a nice day.