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If this is working correctly, you should be able to go to, for example, http://foo.com/www/admin, and your webserver will end up serving the same content in the same way as if you had loaded http://example.com/www/admin - and, because your Revive Adserver installation's var directory has a file called default.conf.php in it, access to the Revive Adserver installation via any domain that it doesn't recognise will be treated as if it came from the domain listed in the realConfig entry of that default.conf.php file - which in this case, will be "example.com" - so, your Revive Adserver installation should still work (using the example.com domain).
Step 2: Copy the default config file to a domain config file
asdfMake a copy of the default.conf.php file in your Revive Adserver's var directory, named according to the domain you want to use, in this case: foo.com.conf.php.
Now, when you access your Revive Adserver installation via the foo.com domain (instead of example.com), you'll still get the same outcome as in Step 1 above - but this time, instead of access doing via the default configuration file (which informs Revive Adserver to use the configuration located in the example.com.conf.php file), access will now happen via the foo.com.conf.php file (which does the same thing - it informs Revive Adserver that the real configuration to use can be found in the example.com.conf.php file).
Step 3: Extend the new domain config file to override the "default" domain settings
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