To add an attachment to a Journal Entry, you will want to do this through the "Activity" function within the Journal Entry.
To add a new activity, right click and choose "New Activity":
Select the type of Activity you'd like to classify it as (e.g. "Other") and then click the Attachments tab and browse to the file you wish to attach, same as any other attachment for a document in Orchestrated:
Once added, right click and choose "Related Activities" to pull up a list of all activities in order to drill in and see their attachments: