Even if you have already added a document via Documents, it is not too late to also add to a specific meeting. The meeting must have already been created and at least one document uploaded to it via attachments.
Go to Documents section.
Click on Advanced Search
Meeting tags look like this.
Drag the meeting date tag to the +tag section
Once you see the background change to green, let go of the tag. Presto the document has now been added to a specific meeting.
To learn how to add documents directly to a meeting - Click here