Modules Tab Greyed Out

I have created a template seemingly following all of the guidelines, however when I get through to the edit email page the modules tab is greyed out and not clickable.

Below is a simple section of the email I am trying to create, containing a 'container' and one 'module'.

Any help or guidance would be much appreciated.

<table width="100%" border="0" cellspacing="0" cellpadding="0" class="mktoContainer" id="mkto-container">

  <tbody>

    <tr>

      <td><table width="100%" border="0" cellspacing="0" cellpadding="0" class="mktoModule" id="subHeading" mktoname="Header">

          <tbody>

            <tr>

              <td height="30" style="line-height: 0px; font-size: 0px;"><img src="elt-spacer.gif" width="10" height="30" /></td>

            </tr>

            <tr>

              <td height="30" style="line-height: 0px; font-size: 0px; border-top:2px solid #008ed6;"><img src="elt-spacer.gif" width="10" height="30" /></td>

            </tr>

            <tr>

              <td align="left"><div class="mktoText" id="text1"  mktoName="text1"><span style="color: #003da5; font-size: 14px; font-weight: bold; text-transform: uppercase; font-family: Arial, Helvetica,' sans-serif'">Trust and Transparency</span></div></td>

            </tr>

          </tbody>

        </table></td>

    </tr>

  </tbody>

</table>