The code is in the Hurd development CVS repository (in libstore/nbd.c).
See http://savannah.gnu.org/projects/hurd/ for how to access that.
But I wouldn't recommend that you try this code unless you are already
comfortable with compiling the Hurd and have (successful) experience with
running your own Hurd.
nbd cannot currently be used for paging on the Hurd. (We only support
paging to portions of local disk, including files on disk.) But the issues
are well-understood here and we already know what implementation work is
required to make it possible to do paging over the network in general.
(i.e., if we did the work necessary to e.g. page to NFS files, that would
also make it possible to use nbd for paging.)