Note: The following will work in SharePoint 2010. For SharePoint 2013 you will need to use PowerShell. See here: http://techtrainingnotes.blogspot.com/2015/02/working-with-quick-launch-from.html
When you attempt to add a link that does not look like a URL to a SharePoint 2010 Publishing site you will get an error message like this one:
The example above works perfectly with a non-Publishing site. What's different? The dialog box being used to do the edit.
The Work Around?
Cheat! Use the old editor for Quick Launch! (This will not work in SharePoint 2013.)
Go to your publishing site and then edit the URL in the browser's address bar so it points to _layouts/quiklnch.aspx
Something like http://yourServer/sites/yourPublishingSite/_layouts/quiklnch.asp
And there you go!
For a SharePoint 2013 workaround using PowerShell see: