IF you are adding a new keyboard to the phone, You should be able turn it on in Settings>General>Keyboard>Keyboards>Add New Keyboard. The newly added Emoji keyboard should appear in the list and you can select it from there.
If you have multiple keyboards and you want to use the new keyboard in the messages app, hold down on the globe or emoji icon and select the keyboard from the window that pops ups. If you only have one emoji keyboard, you would only need to tap the emoji icon to bring up those emojis.
If you are looking to add one of the emoji keyboards that are available in the iMessage section of the App Store, then use the instructions that bobseufert gave you in his post above.