Suppose you have a menu with the following items
and you want to show it as
Then you would create a menu with a reduced number of items. Suppose it is called Mobile-Menu.
Here are the default menus in the demo Joomla site.
Copy the Main Menu using normal Joomla backend commands.
Create the Mobile version of Main Menu.
Then you need to edit the items in the new menu by pulling down and clicking on the new menu.
Unpublish two items.
Click on extensions and a pull down will appear. Click on Module Manager.
You need to be able to see the modules you will be working on, so put menu in the filter.
Joomla tries to be helpful and creates an entry for the new Mobile-Menu, but it publishes it to the pc....
So you need to disable it.
so it looks like this.
You will need to edit all 4 mobile types, but this example is just showing the PDA.
This looks like this. Notice the drop down choice of menus. Also the name on the left is still Main Menu. This is important if you are showing the title of the menu, because it is still the Main Menu for the mobile.
So the menu option is now pointing to the mobile version of the main menu.
Save and you are done.
You can now see the PDA view is using the new menu by viewing the site with a mobile, or an emulator/user agent switcher.
Hope this helps.