MyBB Community Forums

I was looking for a plugin that displays some kind of popup or something else saying "register to view content"

Like you can see on my site, all the content is hidden. Want to tell the guests to register to view

www.glow-team.com

You can just add it to the header_welcomeblock_guest template.

Or search the mods section, there's many plugins that do this.

Important messages is a nice plugin to show your message. You can set which usergroup should see the message
http://mods.mybboard.net/view/important-messages-1.3.7