For everyone searching on this topic and getting frustrated because all the answers are either “give me your admin login” or they’re hidden in the Pro forums – this is how I figured out to customize the page-template-default.php file in my (child) theme:
In FREE version of Download Manager – copy that file from the plugin folder’s /tpls/page-templates/ to your theme’s /THEME-FOLDER/download-manager/page-template-default.php. IE, put it in the root of a new /download-manager/ folder in your theme.
In the PRO version of Download manager – copy that file from the plugin folder’s /tpls/page-templates/ to your theme’s /THEME-FOLDER/download-manager/page-templates/page-template-default.php. IE, put it in a sub-folder called “page-templates” within new /download-manager/ folder in your theme.
This is how I’ve made things work in each version.