I enabled the "Administrators notes" plugin but I am not sure how to access it??
Is it available to all virtual servers?
It looks like after you install it, you may need to enable it.
If you log into Virtualmin, then go into System Settings -> Features and Plugins, there's a checkbox by "Administrator's Notes" that needs to be checked.
After that, if you go to the main Virtualmin screen (System Information), you'll see the "Administrators Notes" all the way at the bottom.