1) You MUST have an SDCard installed BEFORE installing the Amazon app store. The reason for this is that the Amazon app store uses an SDCard to store certain information/cache/etc. While you can can get some things to install without an SDCard, you won't get all your apps to install without one. So if you want to install all your stuff from the Amazon app store, put an SDCard in your Nook FIRST.
2) From your Nook tablet, you can install the Amazon Appstore by typing in the following URL: www.amazon.com/amazonappstoreapp
3) You CAN use the same Amazon app store login on multiple devices. I currently have 2 HTC EVO3D phones and the Nook tablet all using the same login. The advantage of this is that if you (like me) have been grabbing free app of the day for a long time now, you will also have access to these on your Nook tablet.