Add Citations and Files Manually:
If none of the ways to add citations automatically is available, you can manually add an item by clicking on the green "plus" icon above the center Zotero pane:
Then select the resource type and fill in the relevant information (metadata) by hand:
To associate a file with this record, return to the green "plus" icon and select "Link to File":
Start video at 2:34 for instructions on how to add items manually.