Have you created a custom menu? You should be able to do that under a custom menu – drag and drop the items.
http://codex.wordpress.org/Appearance_Menus_Screen
Thanks for getting back.
I added custom menus as described in the link you gave. However, I am not being able to create sub-menus under the current menus. You can see how I got the menus at my site here:
http://www.gadgetgarrio.com
Can I take such sub-menus just by clicking the check box like we add categories as top-level menus?