The "Add" menu has several additional options for adding materials to your list.
Use "Add from favorites" to add any item on your Favorites list to your reading list. See the "Other useful features" section for more information about your Favorites list.
The "Upload files" option allows you to upload your own file, after which the system will pull as much information from the file as it can (title, author, etc) to complete the reading list entry. Make sure to verify all information is correct and make any additions or changes.
"Add using DOI or ISBN" lets you input a DOI or ISBN first, followed by the rest of the item's information.
Use "Import references" if you have a file exported from a reference manager with items you want to add to your reading list. The drop-down menu will tell you the acceptable file types.