If you plan to track unrealised gains and losses, you need to create an income account for this purpose. You may want to name it "Unrealised Currency Gain/Loss," or something similar. We recommend that you consult your accountant to determine whether your business needs to track unrealised gains and losses, and if you do, the most appropriate way for you to do so.
Depending upon your business, you may need to create additional accounts to track foreign cheque accounts, assets held overseas and the like. If you're unsure about the accounts you'll need, your accountant or an MYOB Certified Consultant can assist you with this task.