If the document has already been added via Documents, it can still be added to a specific meeting. The meeting must have already been created.
Go to the Documents section.
Find a document to add to a meeting
This is a meeting tag.
Drag the meeting date tag to the "+ tag..." section
Once the background changes to green, drop the tag. The document has been added to a specific meeting.
To learn how to add documents directly to a meeting - Click here