Well, ... there are two answers to that.
Answer #1: The impulse engines are in a housing under the saucer. The shuttle bay is immediately above where the impulse engines would be in the DD/CA saucer. This is how the 2200/2300/2400 series miniatures show it.
Answer #2: The impulse engines are in the same location as where they are on the DD/CA. The shuttle bay is in the housing structure between the warp engines. The bay door is in the curved surface on the back of that structure. This is how the new 2500 series miniature will show it. For reference, take a look at the pictures
here.
Either one is "correct".