By register i meant was create a roku account and get the device configured on USA isp network. That way device would know it and load apps and its content accordingly. Mine was configured on an Indian ISP network and at that time Amazon prime wasn't launched so since then amazon app is missing from app list.
Yes,that should work.
When you plug the Roku stick to HDMI port for the first time, you'll be asked to log in to Roku account, you'll have to create one since its your first time using a roku device. It will also ask your credit card details (In case you buy any paid apps) but there is workaround to it, search on google. Once account is created, you'll be asked to select the region you are in and then its pretty straight forward process.