Hi Kat T,
Business Central requires a exchange rate service that provides an XML feed. Most of the free feeds out there tend to be JSON based. Try FloatRates.com.

Be sure to set the Transformation Rule for the date field as shown in my screenshot. And be sure to Enable it.
FloatRates also has some custom source features, and I plan to blog about it when I have a chance in the coming weeks.
I hope this helps. If my response has answered your question, please verify by clicking Yes next to "Did this answer your question?"