Try setting this as the URL for your menu item:
javascript:EJEJC_lc('
https://www.e-junkie.com/ecom/gb.php?c=cart&cl=XXXXX&ejc=2')
The XXXXX part would be your client ID. I noticed the example you provided had ID 73425, but you're posting here in the forum from ID 76971, so I wasn't quite certain which one is for your seller's account.
You would also need to have this portion of the View Cart code somewhere in every page, as this calls the programming that generates the nice cart overlay:
<script language="javascript" type="text/javascript">
<!--
function EJEJC_lc(th) { return false; }
// -->
</script>
<script src='
http://www.e-junkie.com/ecom/box.js' type='text/javascript'></script>
You may need to manually edit a template file in order to insert that code someplace where it would appear on every page, and where you wouldn't risk having it mangled by Joomla's CMS somehow. It might also be a good idea to search/ask on the Joomla forum for assistance from anyone else who's used E-junkie on a Joomla site:
http://forum.joomla.org/