I also have a Nook colour HD and all Barnes and Nobel would tell me was to join a library that used Overdrive Media.. Not easy when you live in a country that supports ADE. Adobe, when I finally did get hold of their support was even less helpful
However a friend was VERY helpful. After 3 weeks of trying and me threatening to put the device under a 10lb hammer they suggested the following.
1. Download Overdrive Medial Consolte to your Nook. (search shop and follow instructions)
2. Go on to web on Nook (do not have it connected to your laptop)
3. Connect to your library e page direct from the Nook
4. Choose book (s)
5. Check out
6. Books now appear in Overdrive and are readable.
Result - one very happy lady