This method of sideloading was discovered by a member on the XDA forums, and basically you would have to trick the Nook Tablet into displaying the option. Once the option has been displayed, all you would have to do is check the box that says “Unknown sources” and you’re good to go!
The drawback to this method is that apps will not show up on normal screens, instead you will have to do a search for them if you wish to launch any of your third party apps. This can get tiresome especially if you go on a downloading spree, so it has been suggested that you might want to consider a third party launcher to access those third party apps.
If you’re interested in sideloading third party apps onto the Barnes & Noble Nook Tablet, head on down to here where Liliputing’s Brad Linder walks you through the entire process.