You have to use the item "goobbue horn" and "chocobo whistle" at which point it disappears from your inventory and the corresponding icon appears in your "mounts" tab under the abilities and traits menu. You can equip those icons to your hotbar and simply click them to mount/dismount. If you have done that much and still are unable to call your mounts then there may be a problem. I don't remember if you need to do a story quest up to a certain level to unlock chocobos, however that may very well be the case.