Make sure you have flashed the latest clockworkmod to your nook through the settings of ROM manager, it is 22.214.171.124, on the options on top. The process will take you through it step by step, updating clockworkmod on your nook and then it reboots your nook.
Also make sure to rename your .gapps file to an update.zip without inputting the .zip part, the .zip part is understood already by cyanogenmod. Do not unzip that file. Make sure the update.zip that you renamed from .gapps is sitting on the ROOT of your SD card, and not inside of a folder.
Then use the install custom ROM option from the SD card in the settings of ROM manager, clockworkmod will then start up and you will see the clockworkmod startup screen. choose install from SD card option within clockworkmod. Clockworkmod will then take you through the process of installing the .gapps file which you renamed to an update at that point and reboot. Be patient as it might take up to 5 full minutes for clockworkmod to install all of the binary files, it will then say it will reboot your nook upon completion on its own.
Upon reboot on installing your .gapps file you should get an android startup screen where you input your google email and password, called Gmail, if this is your first time installing your .gapps file. If you do not have a Gmail or its password make yourself one for this purpose. It is free.
If you are updating your .gapps then make sure you go into the application setting of cyanogenmod and delete the data and caches for the android marketplace for it to work properly. The newest .gapps file for your nook can be found here. http://forum.cyanoge...kage-available/
Also make sure you are using the latest stable of cyanogenmod , which is 7.1. The newest .gapps file and the latest stable for cyanogenMod has fixes in them to grant you all access to the available apps for download from the marketplace, google is now calling it "play store" on the icon you should now see in your app drawer or on your home-screens.