I assume you are using a Release Candidate of MODX 2.1?
A new option was added to xPDO which requires you to explicitly set this feature on. Auto-table creation is not always desirable—in MODX, we always explicitly create the tables in the setup process, for new installs and upgrades. Similar would be recommended in any packages you create for MODX.
But you can simply get a new xPDO instance using this option:
<?php
$xpdo = new xPDO($dsn, $user, $pass, array(xPDO::OPT_AUTO_CREATE_TABLES => true));
NOTE: In MODX 2.1, you can manually set options in the config file in the new variable $config_options; these values will be preserved on upgrades.
Or you can use setOption() to change it for the remainder of the current request, at anytime:
<?php
$modx->setOption(xPDO::OPT_AUTO_CREATE_TABLES, true);
Documentation updates are in progress on all of these changes in xPDO 2.1.x and MODX 2.1.x.