To assign them to a task in the checklist, you should select the document you would like to assign and click the Assign to button from the right-side menu. You'll see the list of documents from your checklist. Select the task and your document will be assigned to it. Once the document has been assigned it will disappear from the Unsorted document list.
If the required task is missing in the drop-down list please click on the Create new link, select the required checklist and type the task name. Once you select Save, the document will be automatically assigned to the specified task.