Please make sure you have set correct site url (including https if you have ssl ) from WP Admin => Settings => General
To get client ID & Client Secret you will need to create a connected app in your salesforce account.
Please go to Setup >App Setup > Create > Apps
At bottom you will notice Connected Apps
Now click “New” button in front of connected apps
1- Add Connected App Name and Email Id
2- Enable OAuth Settings and add your site url as Callback Url
3- Add “Full Access” and “Perform requests on your behalf at any time (refresh_token, offline_access)” for OAuth Scopes
You can now copy Consumer Key and Consumer Secret and add it to plugin settings.