I clicked Search, and chose criteria of Task = "" resulting in a list of _all_ tasks (including completed ones).
What is unclear to me now is whether or not the free version of Toodledo gives access to completed tasks that are older than a week. The UI implies that it does (by giving users the option to delete completed tasks after a max. of 6 months), but the marketing literature implies that completed tasks are only available for 1 week. See http://www.toodledo.com/pro.php :
"Full History - A Pro Subscription allows you to view the completed history of your completed tasks (free accounts are limited to 1 week)."