Things You'll Need
Instructions
Rotating Default Screen Display
Exit the game and navigate to the MAME menu.
Select "Video Options" in the menu.
Select "Rotate" until the menu is aligned in the desired direction.
Troubleshooting
Ensure that MAME is configured for cocktail mode. To change between standard and cocktail mode, look in the main menu. Open the "Dip Switch" menu and select "Cabinet Type." In this screen you can move back and forth between upright and cocktail mode.
Check that all the controls are properly assigned for Player 2. Within the game, press "Tab" to access the control menu. Select "Input: This Game" to see how the controls have been assigned.
Check the capabilities of your graphics card. Many MAME arcade tables are powered by older PCs--that's perfectly acceptable because many arcade games are older and not very demanding on a system. However, to be able to rotate the menu, the graphics card needs to support screen rotation.