Probation end dates are set automatically once you have entered a start date for a new member of staff. In order for the system to do this, you need to enter your probation period into your Company Settings.
To do this, go to the Reminders tab of your Company Settings. Scroll down the page until you come to 'Probation' Periods and enter your probation period in months.
NB: This is a company-wide rule, so you cannot set different probation periods.