Open you character screen and your bag. If you purchased/made/conjured food, it should be in the bag. In the Character screen with the Equipment tab open, you may notice two slots on the lower left. If these slots are empty, then you currently have no food or water. Drag the food and water using the left mouse button, from your bag and assign (drop) them to these slots. You will consume these items when you are hungry or thirsty, so you might want to stock up on large quantities if you don't plan on provisioning or purchasing any in the near future. You can only combine like foods and drinks, so you cannot stack Lion Steaks on top of Sunfish Casserole, or Jum Jum Cider on top of Murky Pond Water.