Hi I'm using the latest version of WordPress & your plugin. I have created a custom field, the field is called Steller_Lead__c and it is a checkbox type (values TRUE or FALSE or 0 or 1).
Within the form I have created a CheckBox field, which is hidden and has a single value and followed your instructions to mapping this to Salesforce. When you are trying to map the custom field, I have tried both options of setting the Admin Label to Steller_Lead_c and/or the Parameter Name after selecting "Allow field to be populated dynamically"
When submitting the form, I get a SUCCESS page with a whole page of Array information.
I have looked at other posts and there was due to be a fix for this in the latest revision, however, I seem to be experiencing the same issue without a resolution.