Here are some simple AFL's for the three types of stoploss - Initial, Breakeven and Trailing stoploss.

1. Initial stoploss

/************************************************** **
Initial Stoploss - lowest "low" of the last n bars
************************************************** ***/
lookbackPeriod = Param("Lookback period", 2, 2, 30);
Plot(LLV(L,lookbackPeriod), "Testing", ParamColor( "Long", colorGrey40));

2. Breakeven stoploss

/************************************************** *****
Breakeven Stoploss - Open + brokerage% of open
************************************************** ******/
brokeragePercentage = Param("Brokerage Percentage", 1, 0.1, 5, 0.1);
Plot(O + O*(brokeragePercentage/100) , "Testing", ParamColor( "Long", colorYellow ));

3. Trailing stoploss

/************************************************** **
Trailing Stoploss - a% below nth highest high
************************************************** ***/
lookbackPeriod = Param("Lookback period", 10, 2, 30);
stoplossPercentage = Param("Stoploss Percentage", 5, 0.2, 10, 0.2);

Plot(HHV(H,lookbackPeriod) - HHV(H,lookbackPeriod) * (stoplossPercentage / 100), "Bullish SL", ParamColor( "Long", colorGrey40));
Plot(LLV(L,lookbackPeriod) + LLV(L,lookbackPeriod) * (stoplossPercentage / 100), "Bearish SL", ParamColor( "Short", colorGrey40));

More help at Amibroker itself below,
http://www.amibroker.com/library/
http://www.amibroker.com/guide/afl/