I have the premium version, but I still can´t figure out how to sync my iCloud Calendar in the iphone with todoist.
Please follow the instructions described in this article: http://support.apple.com/kb/ht5029 and let me know in case it won't help,
Those instructions are not working.
Which part is not working for you? Are the tasks not showing up? Are you getting an error when you subscribe to your iCal feed?