can i add an email link as a resource?

You're not doing anything wrong. Custom tabs always go in the topbar.

I can't say for sure why, but my guess is that it's because a custom tab itself doesn't show information the way the Menu or Notes do. Instead, a tab just performs one of the allowed actions: 

So if you want to use a custom tab to lightbox a slide with the course Resources, the tab will have to be in the topbar.