It’s difficult to answer your question without seeing how your menu code is generated; it could be hard-coded, or use wp_list_pages(), or wp_list_categories(), and the solution is different in each case.
Assuming that your menu is created using wp_list_pages(), which is most likely, you can use Page Lists Plus for this. Install and activate, then on the Settings > Page Lists Plus page check the “Nofollow” checkbox and save, then go to the edit screen for the Page that you want to nofollow and check the “Nofollow” checkbox and save.
Alternatively you can do this through your theme files by capturing the output of wp_list_pages() in a variable, doing a str_replace() to add the nofollow, and then echoing the result. If you’d like to do it that way then post back and we’ll go through it.
– Tim