Which MAC computer are you using? If you are using a newer Mac with HDMI, there is a preference panel in the System Preferences that can change the position of your screen. If you are using a Macbook PRO or iMac connected to a TV then you can use this same control panel in System Preferences to adjust screen position and height.
Also, screen position and height can be adjusted directly on your TV screen or external monitor. If you are using a Macbook PRO or iMac and the internal monitor is off center, you may need to reset your Mac or contact your local Mac store genius bar for further assistance.