// You can use `core/session`, `customer/session`, `checkout/session`, `adminhtml/session`
Mage::getSingleton('core/session')->addNotice('Notice Message');
// You can use `core/session`, `customer/session`, `checkout/session`, `adminhtml/session`
Mage::getSingleton('core/session')->addSuccess('Success Message');
// You can use `core/session`, `customer/session`, `checkout/session`, `adminhtml/session`
Mage::getSingleton('core/session')->addWarning('Warning Message');
// You can use `core/session`, `customer/session`, `checkout/session`, `adminhtml/session`
Mage::getSingleton('core/session')->addError('Error Message');
In ..app/design/frontend/base/default/template/page/*
templates
<div class="col-main">
<?php echo $this->getChildHtml('global_messages') ?>
</div>
and app/design/frontend/base/default/layout/page.xml
layout
<block type="core/messages" name="global_messages" as="global_messages"/>