MacJournal saves all of its information to a single file in the macOS Finder. You can sync this file to other devices using file syncing services such as Dropbox or iCloud Drive. To do this, you first need to move MacJournal's data file to the location to where the file syncing service will see it.
If you launch MacJournal again, it should find the file automatically again. This file will now be synced to other machines that are logged into this service.
For best results, only one machine should have MacJournal open at a time. After you make a change and quit MacJournal, make sure the file syncing service has synced the change from that machine to the other machine before opening MacJournal again on the other machine.