To set up the main menu in the tabs position, do the following:

Navigate to Extensions > Module Manager > Main Menu
In the Details section set the Position to hornav
In the Parameters > Module Parameters section set the Menu Style to List
In the Parameters > Module Parameters section click the Yes radio button for Always show sub-menu items. This parameter allows for drop down menus.
If you want to have images displayed on the tabs or buttons then go to Parameters > Other Parameters and click on the Yes radio button in the Show Menu Images.
Click on Save in the top right menu

To display images on the tabs or buttons, do the following:

Upload 16 pixel x 16 pixel images, for each tab or button to the Stories folder in the Media Manager
Navigate to Menus > Main Menu
Click on an individual button such as Contact Us
Open the slide: Parameters (System)
Select the image that you want to use for this tab or button from the Menu Image drop down menu.
Click on the Save icon located in the top right menu
Note: Menu images can be added to horizontal or vertical menus

Modifying Menus

If you want to turn off the tab graphics and have only text links, such as for a main menu in the left or right column, following is the procedure:

Extensions > Module Manager > Main Menu
Parameters > Advanced Parameters > Menu Class Suffix field...type -list. Make sure to use the dash before the word list. This will make the tab or button images disappear. To reactivate the images, delete the -list.
Save
Refresh the frontend