You can tell Mercurial where exactly the hghg command is on the remote machine by using the '--remotecmd'--remotecmd option to 'hg clone'hg clone. See 'hg help clone'hg help clone.
You can also set it in your ~/.hgrc~/.hgrc or the .hg/hgrc.hg/hgrc of a specific repo clone. I think it belongs in the [ui][ui] section.