I figured this out just now, and thought I would share.
Open index.template.php (going through the Admin section is easiest).
Beginning on line 612, find:
<table width="100%" border="0" cellpadding="0" cellspacing="1">
<tr>
<td class="catbg" height="20" align="left" valign="middle" style="color:#ffffff;">Quick Links</td>
</tr>
Add after:
<tr>
<td height="20" align="left" valign="middle" class="windowbg"><a href="', $scripturl, '?action=shop">Shop!</a></td>
</tr>
Within the theme, on the right-hand side of the screen, you'll see the drop-down box for "Quick Links." The shop will be the first one you see, as evidenced on
my forum.