Tech Tip: Getting hgsubversion to Work with subversion-1.7.x
Apache Subversion version 1.7.x broke some backwards compatible and some stuff got broken, including hgsubversion, which provides a way to use subversion remotes with Mercurial.
In order to fix it, just install subvertpy, which provides alternative python bindings for subversion, which hgsubversion prefers by default, and with which it works fine under subversion-1.7.x.
I discovered all that after I attempted to fix it the hard way by installing subversion-1.6.x under /opt/svn-1.6.x, which required building an old version of SWIG, and then setting PATH, LD_LIBRARY_PATH, and PYTHONPATH and then having to remove the global python-svn bindings from under /usr/lib (as root!) because hgsubversion did not like them, a process which took me an entire evening and was frustrating and was ridden with a lot of trial and error, so I would not recommend it.
Hope it helps. Also see this hgsubversion bug report.