You are using PureMart, make sure to create a forum topic in a correct category.
PureMart doesn't support showing percentage of saving. It is also same in the Live Demo of PureMart. But this feature comes with CleanMart though (www.flexiblewebdesign.com/Virtuemart-Templates/CleanMart)
Since this is a 100% custom modification and unfortunately we don't provide free support or step-by-step instructions for custom coding modifications.