To perform a Monte Carlo Analysis, you must use a Monte Carlo alternative type. To do so, select the Monte Carlo radio button on the Run Control tab of the Alternative Editor ("Figure: ResSim Alternative Editor - Run Control Tab - Monte Carlo Alternative Type").

When creating a Monte Carlo alternative, setup a Standard alternative first and get it working.  Then, on the Run Control tab of the Alternative Editor, change the Alternative Type from Standard to Monte Carlo.

Figure: ResSim Alternative Editor - Run Control Tab - Monte Carlo Alternative Type