If you are trying how to get American Netflix on xbox or any other device, you need to be in the US first. That is a no-brainer, but you must note that the content of Netflix is based on geographical location because of regional licenses and censorship. Hence, if you are in any other country, you will not normally get the US content. You can rig your Xbox settings to make it appear like you are in an American location, but that is not a legal thing to do as per the terms and conditions stipulated on the Netflix site.
Here we shall speak primarily about getting American Netflix on the Xbox 360.
According to the instructions given on the Netflix website, you will first need to arrange some settings on the Xbox 360 first. Start with configuring the router if you have one. If there is no router connected to your device, i.e. if it is directly connected to a phone line or to a cable line through a modem, then you will need to set it up differently, i.e. on the Xbox itself.
First you will need to go to your Xbox into the My Xbox menu, and then check out the System Settings. Here you will find Network Settings. In this menu, you have to choose Wired Network. If you do not have a wired network, it might ask you to enter the name of the wireless network you are using. If you are prompted to do this, enter the name.
This will take you to Configure Network. Go to the Basic Settings tab and here you will find the option to mark DNS Settings. Get into this and choose Manual. Then choose Select Primary DNS Server. If you have the primary DNS Server, enter it. You are almost good to go now. Select Done. It will ask you to select a Secondary DNS Server. After putting that in too, you are all set.