You can download the manuscript by:
a) The button 'Download Latest PDF' on the upper right of the review forum. This will allow you to download the manuscript itself. It will also include the figures (if any).
Immediately below this is the Supplementary Materials button; Files in addition to the manuscript pdf/doc files and the Figures
b) 'View all files' will allow you to see if the authors also uploaded supplementary material and download them.
If no mention about supplementary material is made in the field above, the authors did not upload any.