Hi!
Sorry about that, I forgot to look at the screenshot. You are right, it wasn’t working.
The bad news is that I never thought of using the custom content area on multisite installs, this is the first time it came up.
The good news is, since you have provided temporary access I was able to make changes to the plugin files and implemented this feature. Please DO NOT reinstall or delete the plugin, the changes will be lost. This is not a final solution, I had to improvise to make it work for you, so please rather do not use it on your other sites.
I will definitely create a permanent solution for the upcoming version, so no worries about future upgrades.