ReadCube is a separate but related product to Dimensions. When you click "Add to Library" in Dimensions the article citation is added to your ReadCube account. When you login to Dimensions for the first time you will establish a ReadCube account that can be used with Dimensions for this purpose. Once you have logged into Dimensions/ReadCube you will find your citations at https://app.readcube.com.
There is a paid ReadCube Pro version which can be used to manage PDFs and other documents. More information can be found here: https:www/readcube.com. The Caltech subscription to Dimensions provides access to ReadCube (free) but not ReadCube Pro.