Setting up WordPress.com
Create a new WordPress application
Log in to your wordpress.com account and visit https://developer.wordpress.com/apps/
Click Create New Application
Give your application some details
Fill out a name and description for your app. These will be seen by your visitors, so make them descriptive.
Enter in the exact url for your website in the Website URL field.
Enter the Redirect URL that you find in the plugin settings into the Redirect URL field.
Scroll down and click on the Create button.
Add optional additional information (recommended to not freak out your users) 
Grab your keys and get going!
Copy the Client ID and Client Secret into the settings box in Postmatic Social.
When you're ready for the WordPress login to go live, change the Status button to the On position, then click on Save Settings.
That's it!