Best Answer batman† , 20 March 2013 - 02:10 AM
On wp-admin/nav-menus.php , click on the link for Screen Options in the top right of your screen. Hidden away in the dropdown there, below the usual Show on screencheckboxes, is another line of Show advanced menu properties checkboxes.
Just check the box for Link Target and then all your Menu Items will have the additional option of Link Target : "Same window or tab" or "New window or tab"
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