[Plugin: Gravity Forms Salesforce Add-on] Linking form data with Campaign

I’m having the same problem – and I don’t know if this helps, but I found this info in the Salesforce API docs for the Campaign object (“Usage” at the bottom of the page):

The Campaign object is defined only for those organizations that have the marketing feature enabled and valid marketing licenses. In addition, it is accessible only to those users that are enabled as marketing users. If the organization does not have the marketing feature or valid marketing licenses, this object does not appear in the describeGlobal() call, and you can’t use describeSObjects() or query() with the Campaign object.

The plugin uses the describeSObject() call (salesforce-api.php, line 563) to show the available fields for mapping.

Based on the info from the API docs, I’m guessing that the Campaign ID field should show up if you have the marketing feature or valid marketing licenses and are using an enabled marketing user account.