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If your primary goal is to grow your list, then your opt-in box has to be "above the fold"... that is a visitor to your website should not have to scroll down to see it.
But, should it go to the right, left or center? There will be those who tell you that putting it on the right or left converts better than the reverse. But, ultimately it depends on how you will be designing your page and what kind of other content will be there, too.
Often, when all you're going to have is a headline and opt-in box, the box should go in the center. If the content is more robust -- a blog, a sales letter for your opt-in bonus, a video -- putting it to the left or right depends greatly on:
Your best bet is to choose the location that is the most promising and then test it.