You could ask the developer to add functionality to his plugin to display the installment info.
Otherwise, use instructions from the link you provided above to get the script code, etc. and then
you can modify Product Templates
\Views\Product\ProductTemplate.Simple.cshtml
\Views\Product\ProductTemplate.Grouped.cshtml
Or, you can use one of the "HTML anywhere" type plugins (
example in marketplace), as there are many Widget Zones in there - examples
PublicWidgetZones.ProductDetailsOverviewTop
PublicWidgetZones.ProductDetailsOverviewTop
PublicWidgetZones.ProductDetailsInsideOverviewButtonsBefore
PublicWidgetZones.ProductDetailsInsideOverviewButtonsBefore
PublicWidgetZones.ProductDetailsInsideOverviewButtonsAfter
PublicWidgetZones.ProductDetailsInsideOverviewButtonsAfter
PublicWidgetZones.ProductDetailsOverviewBottom
PublicWidgetZones.ProductDetailsOverviewBottom