Currently you can not upload an entire folder into Boardable.
If you would like to upload multiple documents to a meeting then you fill need to open the folder and then highlight the documents you would like to add to Boardable. When all the documents are highlighted you can drag them to the Drag and Drop section to be uploaded.
Check out this FAQ on how to do so: https://boardable.freshdesk.com/support/solutions/articles/22000224610-how-to-upload-multiple-documents-in-the-document-center