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 address
3- Add https://login.salesforce.com/services/oauth2/success as Callback Url
4- 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.
Go to Formidable -> Global Settings -> Salesforce
Enter your client Id and client secret and SAVE it first.
Now click on Authorize Button
After clicking in Authorize button a pop will open.
Login with your salesforce credentials and give access to app.
Now copy the auth code from the url.
Save the copied code in your formidable salesforce settings
Thats it . You should see the Deauthorize button now.