You have a couple of options. If you've uploaded supplementary files, they can be made available from the Reading Tools sidebar, which, once enabled, is available next to the article itself. To enable the RT, log in as a Journal Manager; click the Reading Tools link from your User Home page; and check that they are enabled, and that the Supplementary Files link is also checked.
You can also link directly to the supplementary files by following the directions here
. That last thread also has information on how to upload media files into OJS without using the Supplementary Files option.