Unfortunately the currency viewer is not compatible with Mendix 6 yet. You can contact the creator of the widget to have it updated.
What you can do is use the format string widget where you start with the currency symbol and then show the value behind it as a variable. The format string widget can be downloaded from the app store as well
I have now managed to use Format String to show/view a currency value along with the currency sign. However, what I actually want is to input a value or amend the existing value. From what it seems, I cannot use the format widget within an input reference selector. Any suggestions on this?
You can also have a look at this post for alternatives of the currency-viewer widget.