Okay, people might already know how to do this, but I didn't, and I figure maybe someone else might benefit from this. I didn't see this in any of the previous messages, so sorry if this is repetitive. Also, I discovered this on the Nook Color boards, so credit goes to juanitasJS there.
1. To create a playlist on Nook, when you access your music through the app or through the sdcard or files menu, hold your finger for a few seconds on the song you want to use for a playlist. It'll pop up a menu that with options: Play, Delete, Add to Playlist. Select Add to Playlist. The meny choices are Current or New. Select New, then type in the name of the playlist. Boom, playlist created. Then just press for a few seconds the song(s) you want to add to that playlist, and select Add to Playlist - Name of the Playlist you created.
2. To change the order of songs in a playlist, when you have the playlist loaded, touch your finger to the left or right side of the songs list, and an icon with little lines on it pops up. You can drag the songs into whatever order you wish.
This doesn't work, of course, if you have a shuffle mode on.
Hope this helps