To change a stop loss (or profit target) already in place, hit REPLACE next to the working order in the Order tab.
Of course, you can also just add a stop or limit order to a trade after it has been opened.
If you have a long position and need to enter a brand new stop loss (i.e. you never entered one to start, before opening the position), enter:
SELL TO CLOSE AT STOP XXX
To use an example. Let's say you are long 10 NZDCAD.
To enter a stop loss of 0.865, submit this order:
Sell To Close (STC) 10 NZDCAD @ STOP 0.865
To enter a brand new profit target for a long position, enter:
SELL TO CLOSE AT LIMIT XXX
To use same example above. Enter a profit target of $0.905:
Sell to close 10 NZDCAD @ LIMIT 0.905.