Something like this (untested):
$propertySet = $modx->getObject('modPropertySet', array('name' => 'MyPropertySet'));
$properties = $propertySet->getProperties();
$properties should be a regular PHP associative array containing the properties and their values.
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PLEASE, PLEASE specify the version of MODX you are using.
MODX info for everyone:
http://bobsguides.com/modx.html