In the past you may have encounter problems using MacHg with google code. Since, the servers hosting mercurial repositories on google code do not respond to the Mercurial identify command as other server do. See the full issue here.
However, work has gone into making MacHg work nicely with google code and as of a few revisions ago MacHg appears to fully work with google code. If you are still having problems note the following about repository identities.
See the FAQ item support for hg svn.