I am trying to get the button for a download link to appear in my custom theme. It appears in the default Twenty Fifteen theme (http://labs.cocode.co.uk/wca-wp/2015/03/letter-templates-for-your-mp-and-for-newspapers/?fet_preview=twentyfifteen) but not on my own custom theme (http://labs.cocode.co.uk/wca-wp/2015/03/letter-templates-for-your-mp-and-for-newspapers/). Is there something I need to add to my theme files? Or is it something that is wrong with the plugin?

https://wordpress.org/plugins/download-manager/