Theoretically, slippage is teh difference between the order price and the actual execution price. It can happen randomly in both directions and the effect of it should be neutralised. However, we know (with proves) that the brokers prefer to "apply" slippage only against your benefit. For example, if you open a Long position, the price may slip up and when closing a long position the price may slip downwards. In both cases, you lose the number of points multiplied by the position size.
The EA Studio's Monte Carlo treats the slippage always against your favour. The actual effect of the slippage is as the spread.
EA Studio applies random slippage on the Entry and Exit orders. The program executes the SL and TP on the exact prices.
Anyway, it is very easy and common to simulate the slippage by increasing the spread with several points.
In most cases the negative role of the slippage is neglectable. The biggest problem of the strategies is curve-fitting.
The Monte Carlo includes possibilities to add random changes of the indicator parameters, and I even consider to make these options switched on by default. However, there is no way the future real trading to change your strategy's parameters.
Testing with higher spread is very useful because it virtually includes the negative effect of the slippage.