I have the Uptown Theme and enabled the OnSale Filter for Ajax Filters, but I do not see the checkbox in the filters on any category page. I have several items on sale in my store (littlebitboho.com). I didn't see the filter showing in the demo either. Is it enabled for the demo? Is there another setting that affects this?
I just enabled the On Sale filter from the Ajax Filters settings page in the administration of the Uptown theme demo
I can't access your website right now (it does not load) but just make sure you have products that have Old price set as otherwise the On Sale will never show since there are not discounted products.