Hi richc,
While I haven't used this service before, I can help get you started.
Firstly, you'll want to find a sample of the XML you'll want to generate. I found one here: http://base.google.com/base/intl/en_au/rss-v2.xml
I took that code and threw a foreach around one of the <item> tags, (removing the other <item> tags,) then pasted in a STEP 1 from using the CMSB Code Generator to make a listings page. After replacing some of the example content from the original <item> tags with code to output fields from CMSB, I ended up with this:
<?php header('Content-type: application/xml; charset=utf-8'); ?>
<?php
require_once "C:/wamp/www/CMSB-TRUNK/cmsAdmin/lib/viewer_functions.php";
list($listingsRecords, $listingsMetaData) = getRecords(array(
'tableName' => 'listings',
));
?>
<rss version ="2.0" xmlns:g="http://base.google.com/ns/1.0">
<channel>
<title>Sample RSS v2.0 AU Real Estate feed</title>
<description>A description of your content.</description>
<link>http://www.realestateautest.example.com</link>
<?php foreach ($listingsRecords as $record): ?>
<item>
<title><?php echo htmlspecialchars($record['title']) ?></title>
<g:bathrooms><?php echo htmlspecialchars($record['bathrooms']) ?></g:bathrooms>
<g:bedrooms><?php echo htmlspecialchars($record['bedrooms']) ?></g:bedrooms>
<g:item_type>Housing</g:item_type>
<description><?php echo htmlspecialchars($record['content']) ?></description>
<link rel="alternate" type="text/html" href="<?php echo htmlspecialchars($record['_link']) ?>"/>
<g:price><?php echo htmlspecialchars($record['price']) ?></g:price>
<g:price_type>negotiable</g:price_type>
<g:property_type>house</g:property_type>
<g:listing_type>residential for sale</g:listing_type>
</item>
<?php endforeach ?>
</channel>
</rss>
Note that you'll need to change the filepath to viewer_functions.php (in red above, you can find yours at the top of any viewer page you've generated.) You'll likely also need to change the field names (also in red above) to match your own section.
The above feed only includes the "Required" fields. Once you have that working, you can start adding the "Recommended" and "Optional" fields listed at http://base.google.com/support/bin/answer.py?answer=66779&hl=en_AU .
I hope this helps! Please let me know if you have any questions.
All the best,
Chris