Thanks for responding. Here is the HTML snippet I'm using. I edited the HTML in a text editor then pasted my HTML into the email template. I need my other users to be able to easily edit only the url and text of the button since they are not developers.
<table cellspacing="3" cellpadding="0" border="0">
<tr>
<td height="38" style="font-family:'Myriad Pro',Calibri,sans-serif;padding-top:2px;background-color:#00AFD8;-webkit-border-radius:6px;border-radius:6px;font-size:16px;line-height:1.4;font-weight:bold;text-align:center;white-space:nowrap;">
<a href="http://www.crashplan.com/" target="_blank" style="cursor:pointer;text-decoration:none;padding:.5em 2em; color:#FFF;" class="mktEditable" id="main_button">
<!--[if gte mso 9]> <![endif]-->
Learn More >
<!--[if gte mso 9]> <![endif]-->
</a>
</td>
</tr>
</table>
I also tried putting the mktEditable class on the surrounding td element, but in that case when using the editor in design studio, some of the neccessary components are lost.