If you have tried to add a PlusPack/membership element but it is not showing on the Live Editor, ensure that Optimizepress PlusPack / OptimizeMember has been installed and activated on your site. Activating the plugin should load the missing elements to your current OP installation.
In very rare cases where the issue persists even when the PlusPack or OptimizeMember plugin is active, you would need reinstall PlusPack or OptimizeMember plugin to fix the issue.