Log into the site editor and click the 'Insert' tab.
Under the 'Apps' heading in the left sidebar, choose 'WordPress'.
The widget will appear on the page. Double click on it to bring up the editing window.
Add your WordPress subdomain and click 'Apply'. NB: This is the part before .wordpress.com in the URL.
Click 'View my site' in the top right corner to see how the app displays on the page.