Observed Behavior

If Custom Fields don't appear in the drop-down of certain Activities in Experience Designer, check the configuration of the custom field. It may be a field created under the "CO" package of Service Cloud. These fields must be referenced differently than other custom fields within Service Cloud.

Sample resolution

Let's say you want to reference:  \\ CO \ Incidents \ Custom Field Name 

You would manually type the following reference in the Activity: 

CO$CustomFieldName