Are you trying to use the jq zen cart plugin?
I couldnt find one that worked with v1.5, luckily its really easy to set up anyway, heres how i did it: Download the normal version jqzoom, rename the css and js files so that they began with script_ and style and drop them into the appropriate folders in your template, you then go in and edit the file includes/templates/default_template/templates/tpl_modules_main_product_image.php, add a class to the image link (as per jqzoom instruction) and save it to your template includes/templates/YOUR_TEMPLATE/templates/tpl_modules_main_product_image.php. you then call it with
$(document).ready(function(){
$('.yourclass').jqzoom();
});
put this either in your own javascript file or at the bottom of the jqzoom javascript, you can set various options when calling the script -check out jqzoom docs