Since Confluence 6.14, unfortunately, tasks lists cannot be added via the Diary editor anymore. When using the toolbar function (+) → Task list or the two-bracket shortcut  nothing will happen. Existing tasks, however, are not affected.
This issue came up after Confluence moved to TinyMCE v4 in Confluence 6.14. We have filed a ticket with Atlassian, but this issue remains unresolved to this day.
When editing the whole page, task lists can be added to the individual entries (Content column) in the Diary macro. Those task lists can then be edited with the Diary task editor and marked as resolved as before.