Oh come on, you don't really think that something like that is something you have to unlock somehow, right? If you can't access it it means that it's unavailable at that point in time.
But I need to ask this clarify - Are you talking about replay associated with challenging a play or just regular "video game" instant replay that you can rewind and fast-forward? If it's related to challenging there are certain times when you cannot challenge, such as turnovers, scoring plays or the last two minutes of a half. If a challenge is needed there it will be called by the booth. If it's just normal replay you're trying to get to, you usually have to pause while in the playcall screen - replay won't become available until then.