To log in to Publit.io and use it within WP for inserting media into posts, you will need to:
1. Get API Keys and enter them under the Plugin Settings Page
2. Some browsers can't set session/cookies via Iframe. If this is the case, you will be prompted to Log In to publit.io and to re-open that page.
If you are still having trouble to log in to Publit.io via WP plugins, please see following step-by-step video.