If you have confirmed that your connection to Salesforce is working correctly and you load the S4S Form Mapping wizard to see the following error message: Exception occured when loading Sitecore Forms. Check logs for more information.
If the log file does not have a related error message, then it is likely that the API Controller Type has not been cached.
To resolve this, we have found that deleting the MS-ApiControllerTypeCache.xml file will cause it to be regenerated the cache. The MS-ApiControllerTypeCache.xml file can be found in the C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\root\xxxxxxxx\xxxxxxxx\UserCache directory. You may also need to reset the App Pool for the website.