Well that was quick. An configuration error crept into the release of MacHg 0.9.10 and so I have released MacHg 0.9.11 to resolve this issue.
The issue was that if people had previously used MacHg then in their ~/Application Support/MacHg/ folder there was a hgrc file that MacHg had previously installed. But this file was missing the cedit extension declaration. Fresh MacHg installs didn’t have this problem. Also if your ~/.hgrc has the cedit extension then this problem also didn’t show up.
In any case MacHg 0.9.11 solves this configuration problem. Grab a copy of MacHg 0.9.11 from the usual downloads page.