The promotions that appear on the dashboard are completely random. Some promotions are by region so depending on where you live you may be able to access some and not others.
As far as being able to find a specific promotion, there isn't one. You can simply try to refresh the dashboard to get them to come up but there's no way to simply get a specific one to come up for you. To refresh the dashboard you can press the Guide button
on your Xbox 360 controller and then hold RT (right trigger) and then the Y button on your Xbox 360 controller. The dashboard should refresh and you should get a new set of ad's.
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