Does it show as a Full Drive or as a Music Player i.e.
Is it shown under 'Hard Disk Drives' or under another cateogry i.e. Devices with Removable Storage (or perhaps Music Players).
It should be the latter.
Nook HD+ uses MTP file protocol so it doesn't show as a drive.
It shows as a Device with Removable Storage.
When you double click on it, it shows a folder named 'Internal Storage Folder' and double clicking on that shows files and folders.
Also, it's worth a try to do the following
1) Copy the zip file to your Nook HD+.
2) Unplug and wait for 30-45 seconds.
see if there's an install process that happens.
3) Plug it back in.