FlexGet is meant to be executed the from a user's own crontab, not from /etc/crontab (root). Although this is possible it is highly discouraged.
See http://forum.synology.com/enu/viewtopic.php?f=39&t=13473 for details of how to setup scheduling for users other than root.
Create a new user, crontab. As root, create a file named 'crontab' in crontab's home directory containing:
@hourly crontab /path/to/flexget --cron
This will run FlexGet every hour. You may run it more frequently as well, but I wouldn't recommend going below 30 minutes since it will cause an unnecessary load on RSS feeds and pages you're subscribed to. Some feed providers even ban your IP if you request a feed too often.
To run more often you may use crontab like this:
*/30 * * * * crontab /path/to/flexget --cron
Where 30 is the number of minutes between executions.
Add the crontab to /usr/syno/etc/rc.d/S04crond.sh as described in the link above.
To start or stop cron, use the commands:
/usr/syno/etc.defaults/rc.d/S04crond.sh stop
/usr/syno/etc.defaults/rc.d/S04crond.sh start
Note that cron will need to be restarted after making changes to a crontab.