Hi,
Thanks for using our plugins. I can help out here.
The only way for the automatic updates to work in Multisite is for the plugins to be activated in the Network settings as WordPress does not have a method to update plugins from the subsites in Multisite.
You can continue to use the plugin in multisite, but, you are going to have to manually update the plugins following these steps:
Manual Updates
You can download the latest version from your account here on theeventscalendar.com
Let me know if you have any follow up questions.
Thanks