1. In the dashboard, go to the Menus screen.
2. Click the Screen Options tab at the top.
3. Under "Show Advanced menu properties" click the box to check "Link Target".
4. Then click the down arrow next to the menu item you want to have open in a new tab.
5. There should be a checkbox under the Title Attribute box that says "Open link in a new tab", click to check.
6. Click the Save Menu button to save your changes.