First thing to check would be the fallback country settings under Advanced tab in Misc. box where it says “If country not supported then show:”
Set this to the country you get the most visitors from like US or UK (you must have either Amazon Associate Tag in Amazon settings for that country or have that country enabled in ePN settings).

This option was added in version 1.5.4 and if you are located in a non-Amazon country (or don’t have associate tag for your country) and have this setting set to ‘Nothing’ then you will not be able to see pop ups in the preview.