The price database editor allows you to enter exchange rates between currencies or prices for investments. You can pull it up using the → menu item.
In KMyMoney a “Price” is the price of one currency in terms of a second currency. For example, as I write this, the price of 1 USD in terms of 1 EUR is 0.83; that is, 1 USD costs 0.83 EUR. The first currency is the one being bought or sold, and the second one is the currency used to buy or sell it.
When you press the button, you get the New Price Entry dialog. Enter the first currency (USD in the example above) in the Security field. Enter the second currency (EUR in the example above) in the Currency field. Enter the effective date of the price in the Date field.
KMyMoney will fetch currency conversions from the web. Once you have entered a single price for a pair of currencies, the online quote feature will always list that pair amongst its options.
See the section on Online Price Quotes in the Investments chapter for more details.