While some Islandora content models provide a download link on the object view page, others do not. This requires one or more extra steps.
First, enable the Islandora Downloadable Datastreams module, and configure it under Islandora -> Islandora Utility Modules -> Islandora Downloadable Datastreams. You will need to select the types of objects for which to generate a download link.
Then, enable the Download block and place it in the appropriate page area (Structure -> Blocks).