Creating a tab to link to an external url is quite straightforward. 
Open site setup > zmi > portal_actions > portal_tabs and go to the right hand drop-down, 
leave at CMF Action and hit Add. Add the id (i.e. Blog). Now open the newly created 'Blog'  
and edit it with your info to look like this image

So now that you have entered Title  Blog, Url string:http://foobar.com and selected View under Permissions as well as ticked Visible and saved, you should have a tab which will redirect to an external url!