I think the question might be about apk's received outside the market - ie from a forum etc. I use adb, but is there a way to do it on the NC itself?
uncheck and recheck the "allow non-market apps" box in nook color tools. That should allow you to install. You may or may not need to reboot after unchecking/rechecking. (assuming you used auto-nooter to root and have nook color tools installed)