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).
If your object still does not have a download link, check to make sure that the appropriate datastream is actually present (Manage -> Datastreams).
For Book and Newspaper Issue objects, you will need to generate a PDF datastream first. Manage the object. Find the "Create PDF" button: For Books, it's under the Book tab. For Newspaper Issues, it's under the "Issue" tab.
When the PDF datastream has been created, the download block will appear when you view the object.