Make sure your active element is the root Email, and scroll down to the Webfonts-area.
Click on Search fonts... and begin typing to search for fonts from the Google Fonts directory.
Click on a font to add it.
Click on Add manually. Then supply the CSS Font Family (as it'd be defined in CSS) and the URL (URL of CSS file to load into the email).
After adding a webfont, you can find it from any Font Family -field under Presets:
When you add webfonts in as described in this page, MailDeveloper will automatically wrap your webfont definitions in a conditional block, which makes the fallback to a system font on clients that don't support webfonts. This affects some Outlook-versions mostly.
Without this fallback mechanism those clients would default to Times New Roman when they encounter a webfont. Luckily you don't have to worry about implementing a fallback yourself; MailDeveloper does it for you.