In reality, this is not a supported feature of eCart.
there are a couple of issues i see on your page:
1) the form does not have a name or an ID
<form action="/cart.php?" method="post" >
<form action="/cart.php?" name="eCartMagicBolt2_Display" id="eCartMagicBolt2_Display" method="post" >
2) there is no code to define the MM_callJS function. Select the link in design view, them open the Behaviors panel. in the behaviors panel, edit the MM_callJS behavior, and click OK, this should generate the code for the function.