At the moment I have not tested the field with ACF 5. I will add support when the new version is out of beta perhaps.
As I wrote in the plugin readme, this is intended for developers who want to build with currencies, there’s currently no WYSIWYG feature (except for two simple shortcodes) and I don’t plan to have any.
ACF-wise, the field consists of a select field with a list of currencies. When adding the field to a post using ACF, the user can select one or more currencies. When getting the value of the field from a post with get_field(), it will return the currencies selected. Could be useful if you need to associate one or more currencies to a content. Then you can use the rest of PHP functions provided by the plugin to do something else.