Navigate to the Actions drop-down menu in the campaign you would like to add custom fields to and select Edit Configuration.

Go to the Custom Fields tab in the top menu.  From here you can view, add, and delete any additional fields in your campaign that you wish.

To an additional field, click +Add Custom Field in the upper left hand corner.  Name the field and select the format of the entry from the drop down menu.   

There are a variety of formats to choose from.  You can require everything from a simple text entry to custom lists to select from. 

When you have entered all information for your custom field, click Save.

When you are calling out from the selected campaign, all fields you create will be located in the Custom Fields tab next to Notes