Android by default does not allow apps to be installed outside the market, you have to change the default settings for apps to be installed outside of the market. It is very easy to do. The setting is located in the manage apps.
Also downloading the fiie is not enough, you will have to run an installer to install the app. This could be done with a file manager app like astro.
Also one last point folks have reported problems running the nook for android app on stock nook due to library conflicts. Nobody has reported a problem with this app but i aso think nobody has installed it.
I've installed it just to try t out and it works, but i am not running stock nook on my nook.