Here's how to run ZChecker on Plone skins:

1. Get ZChecker from the Collective SVN and install it into the Products directory of your Plone instance.
2. For best effect also install the mxTidy package. This enables ZChecker to test Page Templates for XHTML compliance.
3. Set the SOFTWARE_HOME environment variable:

   export SOFTWARE_HOME=/path/to/Zope/lib/python
   # Where the core Zope products are
   # On Windows, e.g.: 
   # \Program Files\Plone\Zope\lib\python
   

4. If you have used symbolic links to setup your Plone instance you must also set INSTANCE_HOME:

   export INSTANCE_HOME=/path/to/ZopeInstance 
   # Where your Zope instance lives (var dir etc)
   # On Windows, e.g.: \Program Files\Plone\Data
   

5. Now you can run the !ZChecker tests by typing::

   python CMFPlone/tests/zcheck.py
   # On Windows, e.g.:
   # cd \Program Files\Plone\Data\Products
   # \Program Files\Plone\Python\python.exe@
   # CMFPlone\tests\zcheck.py

Note: ZChecker tests are not unit tests, they only look like they were because the zcheck.py script uses the unit test framework to drive the ZChecker runs. You can also use ZChecker from the ZMI (at least in theory).