The search box also functions as a currency calculator.
This feature is located at the top right of the Moneydance program window.

Simply enter a number followed by a currency ID (such as GBP, EUR, USD, or another currency ID) and the calculator will translate it into all other currencies in your data file.

In the screenshot below, "100 USD" has been entered into the search box.
Currency exchange rates are only available if you have the "Quotes and Exchange Rates" extension installed, or if you have manually entered/imported exchange rates.

Installing extensions and using the 'Quotes and Exchange Rates' extension is detailed within this section of the knowledge base.