You will be able to add or edit the pension plan if you go to Employees -> Choose the employee -> Under Employment scroll down to Pension:
Pick the pension company (either by choosing from the list or write the PBS number).
Choose whether the pension is paid by the employer or the employee - you can also choose both by adding a new line.
Choose whether the pension is paid as a percentage of the salary or a fixed amount.
Still unsure about how to do this? Watch this video: