Are you
- downloading the .acsm token file from the library,
- opening it on ADE on the Mac (thus downloading a DRM .epub file that contains the excyrpted book)
- transferring it to the Nook from within ADE? (that will copy the .epub file, not the original .acsm)?
That is normally the suggested route.
If it doesn't work, it may be worth using ADE to get the .epub file, then copying the .epub file to the Nook using Mac Finder instead of ADE.
Have you checked time zones etc on the Nook as well?
Have you authorized the Nook from ADE?
To make double sure, it could be worth de-authorizing it (cmd-shift-E to ADE while Nook plugged into the Mac) and re-authorizing it.