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://[email protected]//home/pfeifer/repositories/pfeifer/ns-3-para