Just looked into it, apparently this tech was originally from XDA devs and the N2A people just ripped it off and charge people for it.
There is some info here:
http://taosaur.blogspot.ch/2011/09/r...m-sd-nook.html
The original thread is like 175 pages long, and I know they don't like support questions in dev threads over there, but there is probably a support thread somewhere.
http://forum.xda-developers.com/show....php?t=1000957
EDIT: Continuing to look around I noticed another thread where someone already pointed you at that info. Anyway if your card doesn't work for you, you might want to try that method. E.g. there is another step for gapps.