Setup

First you must enable the currencies you want to use and the default currency that you want the exchange rates to be calculated from. 

To view this list you must navigate to Control Panel -> Currencies.

From here it is possible to edit the currency by double clicking on the row you wish to edit. This editor is mostly read only but you can change the "Exchange Rate", "Default" and Enabled" properties.

To disable the currencies auto updating you can change the system setting "Auto Update Currency" to false. The system settings are found in the Control Panel -> System Settings.

From here it is also possible to add a "Currency Offset Multiplier" which is number which all currencies (apart from default) will be multiplied by. This is used if you want to charge extra for using a different currency. For example a multiplier of 1.05 will give a 5% increase to all currencies that arent the default.

Extra

The currencies are updated using this xml http://www.ecb.europa.eu/stats/eurofxref/eurofxref-daily.xml