From Lunarpages Web Hosting Wiki
Crontab will allow you to automatically run scheduled tasks on the server, and if selected, receive automatic emails showing the results.
How to Use Manage Crontab in Plesk
First step would be to login to Plesk, and click on the domain name you wish to add the crontab for. Now, go down to the Crontab icon and click it. Next, you will need to select the name of the user for whom you wish to manage the crontab.
You can also setup crontab messages to be sent to your e-mail address.
To get the task setup, click on the "Add New Task" icon under your Crontab tasks page. On the next page you should see a web page that reads "Editing the Crontabs tasks of yourusername". Enter the specific parameters to set the frequency of the task. Enter * to select all.
Now under command, you will enter the path to the fill that will execute the crontab. Once that is done, hit the "OK" button and you have finished creating your first crontab on your hosting account.