You have to purchase Mountain Lion from the App Store - you were attempting to install an update. There are no further updates for your version of Snow Leopard. OS X 10.6.8 is the final one.
If your MacBook does not meet Mountain Lion's system requirements you can phone Apple on the number in the Apple Online Store and purchase a download code for Lion instead. "Redeem" this code in the App Store to download Lion.
Mac OS X 10.8 "Mountain Lion" System Requirements
To install Mountain Lion, you need one of these Macs:
- iMac (Mid 2007 or newer)
- MacBook (Late 2008 Aluminum, or Early 2009 or newer)
- MacBook Pro (Mid/Late 2007 or newer)
- MacBook Air (Late 2008 or newer)
- Mac mini (Early 2009 or newer)
- Mac Pro (Early 2008 or newer)
- Xserve (Early 2009)
Your Mac needs:
- OS X v10.6.8 or OS X Lion already installed
- 2 GB or more of memory (RAM)
- 8 GB or more of available hard disk space
If your system does not support Mountain Lion's requirements, you may be able to upgrade to Lion. To do that you must call Apple to order a download code. Redeem the download code in the App Store to download Lion.
Make sure your system meets the requirements for Lion:
Mac OS X 10.7 "Lion" System Requirements
- Mac with an Intel Core 2 Duo, Core i3, Core i5, Core i7, or Xeon CPU
Core Duo CPUs are not sufficient for Lion
- 2GB of memory (RAM)
- OS X v10.6.6 or later (v10.6.8 recommended)
To determine which Mac you have, what kind of CPU it uses, and how much memory is installed: > About this Mac