Hello, to get both the plugin and API key please visit DropshipMe site. Click this link, enter your email and click Get my plugin now, then click Continue on the top of the next page. Then you'll see Thank you page with your personal API key and a link to the zip file with the plugin.
Besides, the plugin can also be installed right from the WP admin area as it has been added to official Wordpress.org plugins library.
Go to Plugins - Add new and enter "dropshipme" in the search field. Click Install Now button, then click Activate.