Install from unknown sources on Nook Tablet
As I wrote in my above comment, one way would be:


Get ADB working on your computer (see: Is there a minimal installation of ADB?)
Download the .apk file to your computer
Switch on USB Debugging on your device, and then connect it to your computer via USB (order is important here: first enable the option, then connect)
Use adb install &lt;file_name.apk&gt; to install the app


For subsequent installs, you of course can ommit the first step, as ADB should already be set up then :)

Alternative approach (untested):


Install some terminal app on your device (e.g. Android Terminal Emulator)
Save the .apk to your SDCard
Start the terminal app, and run pm install &lt;/path/to/filename.apk&gt;


Again, the first step is only needed once. I've not tested that second method, so I'm not sure whether it requires root or the "unknown sources" option. As adb install only seems to be a wrapper to pm install, the latter is not that likely.