Many people are using Elegant Themes eStore template, this should help you as well.
Problem: Add To Cart button is visible but does nothing, has no action.
Fix (All steps must be followed):
1. Product must have unique SKU. (make sure you didn't use that SKU on any other post or page or any past product that you deleted).
2. Product must have a price both under ET settings AND option field (even if you product doesn't have multiple options, enter a value there regardless).
3. Product must a description.
4. Product must have "Stock Available" checked.
5. eStore cart plugin must be set to Live mode (by default it's set to test mode)
6. Product must have a unique URL. If your product shares the same URL as any other page on your site, the Add To Cart button will not work and may display a '#' sign and do nothing. To fix that change the products URL to one that you are SURE never existed before. IMPORTANT!!! Even if you use a URL of a page you deleted long ago, it will still conflict, so please make 100% sure the product has a never before used URL.