Hey Ian,
Thank you for reaching out to us! I’m very sorry about this issue you are experiencing, let me help you to solve it!
At this moment I’ve ran some tests in my local WordPress install and couldn’t reproduce the problem: if I access my site as an editor, I can see and select all venues and organizers previously created by my admin user.
There’s probably a plugin modifying the way WordPress grant user permissions or a custom code in your theme’s functions.php. It is also possible that there is a template override modifying your roles and permissions. To get started, I would suggest you to run a Conflicts Test to further investigate.
If the issue persists even after disabling all of your plugins (except Modern Tribe plugins) and changing your theme to another one like Twenty Seventeen, I would suggest you to check the following article and change your editor permissions using a plugin like Members plugin:
Editing User Roles and Permissions
I hope this helps you get started! Let me know if you need anything else in the mean time and I’ll be happy to assist!
Best regards,
Patricia