The app may not work if you have or had other Currency Converter app installed. Usually, they leave behind extra code that lead to conflicts. Both scripts, ours and the ones in your theme are consecutively adjusting the prices in some cases. Also, you should know that our app never modifies your theme file.
Until that code is removed no currency converter will be able to function properly - so you can either start with a clean version of the theme or ask a developer to look into this. #1 it's always much better, we always advise our clients to have a backup of their theme before starting installing apps, this way you can easily roll back to a good version if things go south.
You can contact us to look into this for more details.