Sometimes you need to add more than one currency code and covert between them like from AED to USD then you need to setup the currencies and conversion rate between them do the following :

  *  General Ledger Module-->From Setup -> set Exchange Rate




* From Currency converter (tab) Check the Check Box(conversion) to apply converting from this currency to another currency


  *  Set the exchange rate for each Currency  

* here you will put the exchange rate and you will see the what you did in down bar 

then save an d close when you finish all the conversion