You can set the respective trading commissions here.
Slippage - if you are trading on a dynamic market, the current rate can change between the time of sending an order to a broker and the time of receiving it. In that case, the broker sends back an enquiry as to whether you accept the new price. If this is often the case, you can set a few pips slippage. Forex Strategy Builder accrues this slippage both, when you open and close a position.