Good question Aaron,
WordPress loves to hide things. Go to the Appearance section and select Menus. In the top right hand corner, click on the “Screen Options” tab and make sure the box next to “Link Target” is checked.
Then look down at your navigation menu. Find the first item you’d like to open in a new window and click the down arrow on the right hand side of it. That will open up the options for that item. Check the box next to open link in new tab/window. Repeat that for all the items you’d like to open that way and save your menu.
That should do it. If you run into any trouble, let me know.
Thanks,
Bill