As Sunfire said, your best bet is to load an eye texture from the library. If you need to do it manually, you first need to be on the eye texture.

If you'll look where I've underlined, you'll see you have the inner mouth selected. Click where I have my arrow to select the correct material zone. You'll want to look for iris. You'll probably need to update the pupil and sclera too as image maps sometimes bleed over.

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