Plesk 10 has an undocumented 'feature' which requires the domain's FTP user have a real shell (i.e. SSH access), or else many common commands won't work in cron. To enable this:
1) Log into Plesk
2) Click on 'Domains'
3) Click on the 'control panel' link for the domain for which you wish to enable SSH for the FTP user
4) Click on 'Websites & Domains'
5) Click on 'Web Hosting Settings'
6) Select '/bin/bash' in the drop-down next to 'SSH access'
7) Click 'OK' to save your changes
Example of curl:
Change domain.com to your domain name.
Also cron.php should have the appropiate server rights.
sudo chown apache:apache cron.php gives the server access to run the cron script.
Remember also to have the includes directory with the same access rights.