Currently Being ModeratedFeb 14, 2013 10:38 PM (in response to Sea-chel)
See > http://www.apple.com/osx/specs In that page, you can see the OS X Mountain Lion requirements. From what your screenshot says, your Mac isn't compatible with OS X Mountain Lion, so you have an older Mac than the ones that can run Mountain Lion.
Unfortunately for you, you can't upgrade to OS X Mountain Lion. However, you can upgrade to OS X Lion if you want, so call Apple to purchase OS X Lion > http://support.apple.com/kb/HE57 Before upgrading, make a backup of your files and check that your applications are supported > http://www.roaringapps.com