Search for SEO Smart links plugin from Prelovac. I have tested it and works brilliantly. It will allow you to put rel="nofollow" and target="_blank" to the links. For those who don't know, target blank will make the link open in new tab/window (depending on browser) while the current window stays untouched.
1. Click on the HTML view of your WP post/page. 2. Select the link which you want to be nofollowed. 3. Just before the attribute within the double quotes in the href tag ends, add this line - rel="nofollow" 4. Thats it. Publish your post/page and the external link will be nofollowed.
Uncle Sam I don't want to do that to all of my external links cause it'll end up being time consuming. What I needed was a plugin to make all external links to have nofollow tag