Let me start by saying that I don't believe this plugin works properly with 3.2. When submitting the form the "Error message when Salesforce.com connection fails" is displayed. However, everything is entered in to Salesforce correctly. A quick fix for this was placing "Success" text into that field.
I also needed to submit a hidden field to salesforce for sorting purposes, in addition to Lead Source. I was able to achieve this by placing additional $post code into line 395 of salesforce.php.
$post['field-id-from-sf'] = 'field-value';