This is the one I use, written by Nicolas. It assumes a fixed stop loss to start with, so you have to add SET STOP PLOSS x after the line of entry code.

//trailing stop function
trailingstart = 10 //trailing will start @trailinstart points profit
trailingstep = 5 //trailing step to move the "stoploss"
 
//reset the stoploss value
IF NOT ONMARKET THEN
newSL=0
ENDIF
 
//manage long positions
IF LONGONMARKET THEN
//first move (breakeven)
IF newSL=0 AND close-tradeprice(1)>=trailingstart*pipsize THEN
newSL = tradeprice(1)+trailingstep*pipsize
ENDIF
//next moves
IF newSL>0 AND close-newSL>=trailingstep*pipsize THEN
newSL = newSL+trailingstep*pipsize
ENDIF
ENDIF
 
//manage short positions
IF SHORTONMARKET THEN
//first move (breakeven)
IF newSL=0 AND tradeprice(1)-close>=trailingstart*pipsize THEN
newSL = tradeprice(1)-trailingstep*pipsize
ENDIF
//next moves
IF newSL>0 AND newSL-close>=trailingstep*pipsize THEN
newSL = newSL-trailingstep*pipsize
ENDIF
ENDIF
 
//stop order to exit the positions
IF newSL>0 THEN
SELL AT newSL STOP
EXITSHORT AT newSL STOP
ENDIF
//************************************************************************

You might also try this MFE code, also by Nicolas:

//trailing stop
trailingstop = 20
 
//resetting variables when no trades are on market
if not onmarket then
 MAXPRICE = 0
 MINPRICE = close
 priceexit = 0
endif
 
//case SHORT order
if shortonmarket then
 MINPRICE = MIN(MINPRICE,close) //saving the MFE of the current trade
 if tradeprice(1)-MINPRICE>=trailingstop*pointsize then //if the MFE is higher than the trailingstop then  
  priceexit = MINPRICE+trailingstop*pointsize //set the exit price at the MFE + trailing stop price level
 endif
endif
 
//case LONG order
if longonmarket then
 MAXPRICE = MAX(MAXPRICE,close) //saving the MFE of the current trade
 if MAXPRICE-tradeprice(1)>=trailingstop*pointsize then //if the MFE is higher than the trailingstop then
  priceexit = MAXPRICE-trailingstop*pointsize //set the exit price at the MFE - trailing stop price level
 endif
endif
 
//exit on trailing stop price levels
if onmarket and priceexit>0 then
 EXITSHORT AT priceexit STOP
 SELL AT priceexit STOP
endif