Another tip
For those of us that got here by Googleing, here's what finally let me install the Amazon app store:
1.) Root your nook with 1.2 software version using ManualNooter.
2.) Uncheck and re-check the "Allow Non-Market Apps" in "Nook Color Tools".
3.) Power off and power back on the Nook Color.
4.) Click the link in the Amazon email. If you've already installed Opera, or another browser, make sure to complete the action using the Nook's built-in browser to avoid the need for an SD card. I didn't try it with Opera, but it should work, as long as you've got your SD card installed. (You'll have to pick this 4 or 5 times, once for each redirect).
5.) Once the .apk downloads, the download icon in the lower left notification area will stop moving, tap it, then tap the .apk.
6.) It will ask you again to choose how to complete the action. Choose the bottom one. It is called "Package Installer". You can set this as default, because the top one "App Installer" NEVER works.
7.) Pick install, and enjoy!
If you didn't set it as default, you'll have to make this choice each time you install something from Amazon. For some reason, Amazon App Store also requires you to have an SD card installed to download anything.
Hope that helps!