You first want to define a second menu that lists your categories. Montezuma supports two different custom menus. If you go to Appearance > Menus and click on the Manage Locations tab, you’ll see an entry for Menu 1 and Menu 2. So what you’ll want to do is create a new menu listing just your categories and select it for Menu 2.
To create a menu of category names, click the Edit Menus tab, then the create a new menu link. Give a name to your new menu and click the Create menu button.
On the left, under the box that’s labeled Pages should be a section labeled Categories. If you do not see it, then click on the Screen Options tab in the upper right corner of the page and check the box labeled Categories. You should now see a section labeled Categories under the section labeled Pages on the left.
Expand the Categories section, then select all of the categories that you want to appear on the menu and click the Add to Menu button. Arrange the menu items in the order that you want them to appear, then click the Save menu button.
Click the Manage Locations tab and select your new menu as Menu 2. Click the Save changes button.
Now go to your header.php virtual sub-template. Look for the function wp_nav_menu. You’re going to want to copy the code for that function and paste it under your logo. You’ll need to change each instance of the number 1 to 2 in your new copy. For example, change menu1 to menu2 for the menu_id and theme_location, and change menu1-wrapper to menu2-wrapper for container_id.
That should do it. The only thing left would be to add any CSS to style the new menu the way that you want.