The Account settings dialog box allows you to set the parameters of your virtual account. You can open it from Account → Account Settings menu command of Forex Strategy Builder.
Account Exchange Rate - you can use it to convert the trading result from the trading currency into your account's currency. If you are trading EURUSD and your account is in USD, no conversion is necessary because the profit or loss you have made is in USD. If your account is in EUR, Forex Strategy Builder uses the Close price of each bar to calculate the result from the trading you have done in USD. If you are trading USDJPY and your account is in EUR, Forex Strategy Builder uses a fixed exchange rate. In that example the account exchange rate will be the rate EURJPY. You can select the last market rate here or to use an average rate for the period covered by the historical data you use.
Forex Strategy Builder saves this settings in a configuration file. They apply to all strategies.