Looking at your screenshot, it seems like you have not assigned any uncertainty values to the flows in the Inputs/Outputs list of the processes that belong to your product system. Some databases ship with pre-assigned uncertainty values, others do not. In the latter case you must assign values yourself. Just insert them in the column 'Uncertainty' of the Inputs/Outputs list of a process.
Update: You do not see any variation because your standard deviation is 1. If you enter e.g. 5 you will see variation during the MCS.