raptus.mercurialstorage depends on Products.ExternalStorage 0.7 which has some flaws and needs to be patched in order to work correctly. The patch may be applied by using the following part in your buildout:

[patch_es_storage]
recipe = iw.recipe.cmd:py
on_install = true
on_update = true
cmds =
    >>> import os
    >>> patch = os.path.join("""${buildout:directory}""".strip(), 'eggs/raptus.mercurialstorage-0.4-py2.4.egg/raptus/mercurialstorage', 'es.patch')
    >>> file = os.path.join("""${buildout:directory}""".strip(), 'eggs/Products.ExternalStorage-0.7-py2.4.egg/Products/ExternalStorage/ExternalStorage.py')
    >>> if os.path.exists(file):
    >>>     os.system('patch -N %s %s' % (file, patch))

You also need mercurial working copies at the root of your storages so if you have a field on which you use an ExternalMercurialStorage with a prefix of foo make sure a folder with the name foo exists in your instances var folder and is initialized using 'hg init'. If you are using buildout, there are two var folders, one in your buildout's root and one at parts > instance, the aforementioned folder has to be created in the latter one.

All the mercurial commits are handled asynchronous using a queue, as a side effect the files saved to mercurial storage are not immediately visible in plone, in this case a attribute called 'asynch' is set on the content instance, which may be used to display an information message to the user. The mentioned queue is processed by a view named 'processMercurialActionQueue' which has to be called regularly by a user having the 'Manage portal' permission. It is suggested to use zope clock server to call this view about ever minute or more, add the following configuration to your instance part's zope-conf-additional variable:

<clock-server>
  method [PATH_TO_YOUR_PLONE_SITE]@@processMercurialActionQueue
  period 60
  user [MANAGER_USERID]
  password [MANAGER_PWD]
  host localhost
</clock-server>

raptus.mercurialstorage logs to a different log file which is found in your log folder under the name mercurial.log.

Products.ExternalStorage stores absolute file paths in the ZODB which makes moving existing plone instances a pain. This issue was fixed in version 1.0b3 of raptus.mercurialstorage. To migrate an existing plone site use the migrate script located in the scripts folder of the raptus.mercurialstorage egg. Information about the usage of the script are given by executing:

bin/instance run path_to_the_script -h