To clone the parallelized ns-3 branch you can type the following:

hg clone http://code.nsnam.org/pfeifer/ns-3-para

# after some days you can update the repository via
hg pull http://code.nsnam.org/pfeifer/ns-3-para
# and update the local copy
hg update

BTW: to push the changes upstream type:

hg -v push ssh://pfeifer@code.nsnam.org//home/pfeifer/repositories/pfeifer/ns-3-para