Salesforce custom objects aren't being populated in the mapping screen
Salesforce custom objects aren't being populated when creating a new mapping. Also, clearing the cache by clicking the Click Here hyperlink above the Add button doesn't solve the issue.
This might be due to the Salesforce integration user's lack of permissions.
Make sure the Salesforce integration user has Read, Write, Edit permissions to the custom object.
To check and set permissions, go to Setup > Profiles > choose the integration user profile > check the Custom Object Permissions section.