Hi Charles,
You can overwrite the default config of T3 by follow these steps:
1. Open \templates\t3_blank\templateDetails.xml and add these xml inside <fields> ... </fields> tag.
<fieldset name="navigation_params" label="T3_NAVIGATION_LABEL" description="T3_NAVIGATION_DESC">
<field name="navigation_type" type="list" class="btn-group" default="megamenu"
label="T3_NAVIGATION_TYPE_LABEL"
description="T3_NAVIGATION_TYPE_DESC">
<option value="joomla">Joomla Module</option>
<option value="megamenu">Megamenu</option>
</field>
</fieldset>
Open \templates\t3_blank\tpls\blocks\mainnav.php
Change
<?php if ($this->getParam('navigation_type') == 'megamenu') : ?>
to
<?php if ($this->getParam('navigation_type', 'megamenu') == 'megamenu') : ?>
Hope it helps,
Regads