Select Extensions - Module Manager. Select New then click on Menu (or select it and click on Next).
Enter a title (yes - again!)
Decide where you want your module to appear (top, user1, left, right - you need find out what areas are available to you on your template - one method is to use http://www.yoursite.com/?tp=1
to show the areas available) (rememenr my PS - if using simple template - use right instead of left!)
Click on Menu Name and select the one you created earlier. Finally you can now select where the menu appears - to do this goto menu assignment and select "Select Menu Item(s) from the List".
Now click on which mainmenu item you'd like your new menu to linked to. (If you want your menu to appear on the any Parents menu pages, click on that one - click on as many as you like)
Click on Save and then check that your menu appears on the page(s) you wanted it to.