I understand you are trying to have a menu item available a top level that point directly to the Tiki Webmail.
By default, using the core Tiki menu (#42) you should be able to see it under "My account" (the user account area). But this menu is not customisable so not good for moving the Webmail item to the top-level.
Anyway, for everyday usage, I recommend you to create your own (user) menu with what you need display like you want. See: https://doc.tiki.org/Menu
Managing your own menu is super handy as you can modify/add absolutely anything; position, submenus, icons, colors, etc. You can also use a Tiki module inside the menu item to display dynamically content (like a category list that will point to pages), have wiki structure, etc.
As admin you still need the core menu (#42) but I reserve it for my administration only. I usually place in the bottom modules section with "admin" can view only. Check also my tutorial about this https://youtu.be/5fnFjNj4-Ss?t=312 - after 5mn.
Hope this helps.
Just a note about documentation, Tiki is an open source project and the documentation is made by the users and anyone can contribute and improve. Don't hesitate to add your input there.
