power store is a prebuilt application, we do not support making modifications to it.
especially since the modification you are trying to make is functionality that already exists in the power store products menu.you should look at the existing menu for an example of now it is accomplished.
also, on the CSS Menu Writer support page, there is a tutorial for creating dynamic menus that will help.
if you require further help making it work, you should sign up for a premiere support appointment.