Modify Main Menu Custom Links in WordPress 3.0 To Open In New Page?

For the upgrade to WP 3.0 I have chosen to change to a common theme for my blogs, so at the moment I am changing them over one by one in a test environment before taking them live on the active URL.

For the current blog I am working with, I have several related sites like an Amazon shopping site that I link to in the main menu bar, but I have them opening on their own tab or window.

Under Appearance – Menus in the Admin panel, there is an option to add Custom Links. There is not the option, however, to choose whether these links should open in a new window or not.

WordPress 3.0
Theme: Magazine Basic 2.6.8
Test Site: http://www.imperfectyou.com/imytest/

Shop Amazon Link in the Top Menu is added using the Page Links To plugin. In the Top Menu, the ability to open in a new window is blocked. In the pages list on the left sidebar it will open in a new page.

The Amazon Shop link in the Top Menu is added using Custom Links and of course it can not open in a new window either.

How do I either add this as an option or manually change a link added here to open in it’s own window/tab? Why is the pages links to plugin functionality blocked?

Additional Info: According to function.php, this theme uses wp_nav_menu

If I need to post anything from the code to assist in resolving this issue, please indicate what I should look for.

Thank you