First use the autoinstaller first to see if it gets NITdroid working.

After that you'll need to mount the android partition and replace the NITdroid files with Gingerbread: http://downloads.nitdroid.com/e-yes/gingerbread.tar.bz2.

I suggests that you use the autoinstaller now and try it first.

Everything you now have on the memory card will be replaced with NITdroid. Make sure you don't have anything important there.