Go through the HTML and make sure the <table> has the width attribute to 100%.
Then, align the <td> content with the align attribute.
There may be a lot of nested tables and you'll need to go through all of them.
<table width="100%" border="0" cellspacing="0" cellpadding="0">
<tr>
<td align="center">
Your Content
</td>
</tr>
</table>