middle = sma(period) (some are using ema but in the original Chester Keltner used sma of typical price)
upper = middle + atr(period)*atr multiplier
lower = middle - atr(period)*atr multiplier
@Mladen Rakic correctly describes how to calculate the values internally within your EA.
Additionally, you could pull the values from an indicator using iCustom().
Just for information -
Small thread with the EA (EA is based on keltner channel and macd and should be fixed to work with MT4 600+ builds so it is for Freelance for example).
Pyramiding strategies(EA was uloaded on the second post of the thread)