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)
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?
If that is the question use ASTRO, navagate to downloaded apk, install See: http://www.androidtablets.net/forum...rooting-information-read-first.html#post19616
i confirm this - this is how i got mine to work - did not recall needing to reboot outside of market, easy way to get apps to your nook - get Awesome Drop, and you can wirelessly transfer apks from pc to nook color have tested it successfully up to 25MB file size