First go to the plugin's admin page.
In the section Page-Specific Options
make sure you have at least this one checked
Alternative Link Text
Now open a page you want to change the menu text of in the WP page editor.
Scroll down until you see the Page Lists Plus section below the editor box.
In the Alternative Link Text box, enter in the text you want displayed in the menu for that page
Save the page (Update) and when you refresh the web page you should now see the text you entered in that box on the nav.