我是I-485结婚green card （ i am in US）。 Just got online：
On February 14, 2012, we mailed you an appointment notice for an interview for March 20, 2012, at NATIONAL BENEFITS CENTER. Please follow the instructions on the notice. If you move while this case is pending, please use our Change of Address online tool to update your case with your new address or call our customer service center at 1-800-375-5283.
Q：
我要做什么准备？
where is NATIONAL BENEFITS CENTER？
Thank you very much