For PERT to work correctly, you must enter the optimistic, expected, and pessimistic durations for your tasks. The easiest way to do this for multiple tasks is to switch to PERT entry sheet view using the Views menu or the PERT toolbar. Then, you will see this window:

 

 

Now, you must enter the values for each task. If the dates are going to be the same for multiple tasks, or if you just want to edit one task, you can select those tasks and click the PERT Entry Form button () on the PERT toolbar.

 

 

You can then use the dialog seen above to enter the durations for each scenario.